A meeting at the Merseyside Maritime Museum open to anyone interested in the tides and the port of Liverpool.
This meeting is organised by the National Oceanography Centre and the University of Liverpool, in association with the Centre for Port and Maritime History (University of Liverpool, Liverpool John Moores University and Merseyside Maritime Museum) and the Liverpool Institute for Sustainable Coasts and Oceans (National Oceanography Centre, University of Liverpool and Liverpool John Moores University).
This meeting marks the 100th anniversary of the world-famous Liverpool Tidal Institute, founded at Liverpool University in 1919 before moving to Bidston Observatory on the Wirral.
Click here for more information, including agenda, list of speakers and how to book.
In 1969 Bidston Observatory became a component body of the Natural Environment Research Council and was renamed the Institute of Coastal Oceanography and Tides (ICOT) with an expansion of its oceanographic work. In the ICOT Annual Report for 1969/70 it states:–
“An essential component of any environmental research effort is the acquisition of relevant observations against which theories can be tested. In the marine sciences such fieldwork is invariably expensive both in capital equipment and operating costs; data acquisition systems should therefore be designed for maximum efficiency and minimum maintenance. It follows that such a system will provide a basis for the long-term monitoring of oceanographic variables, the analysis of which can be expected to yield a bonus in the same way that barometers and thermometers have contributed to both synoptic meteorology and climatology.”
“Dr Skinner was appointed head of the new instrument section in January (1970) and has commenced a critical survey of sensors, instrumentation techniques and data acquisition systems currently available in the field of coastal oceanography. Progress has been made in equipping design and maintenance facilities, and special attention is being paid to test and calibration facilities for transducers and specialised instrumentation.”
I joined ICOT in 1971 after ten years at the Atomic Energy Research Establishment at Harwell. On arrival at Bidston I was given a desk in what was originally the Morning Room of the Observatory which had been divided into two offices. One of these offices was occupied by Len Skinner, then Head of the Research Technology Division, and the other I shared with Ivor Chivers and Judith Daniels. Other technology staff, Alan Harrison, Alex Kerr, Tony Banaszek, Chris Walker, Bev Hughes and Doug Leighton, who was at that time the Tide Gauge Inspector, occupied rooms in the basement of the Observatory originally called the Cellars. In 1972 a prefabricated hut was erected on the front lawn of the Observatory to house the mechanical engineering workshop which was supervised by Kevin Taylor.
The Research Technology Division was organised into three sections, the Instrumentation Section, the Mechanical Design and Engineering Section and the Marine Operations Section. As well as design, development and deployment of new and improved oceanographic instrumentation and equipment, the Division was responsible for the maintenance, calibration, deployment and installation of existing systems. This involved most members of the Division in sea-going activities for the deployment, recovery and use of oceanographic equipment. In the 1971/72 ICOT Annual Report it states:–
“In the past year all three sections of the Research Technology Division have been engaged in an extensive programme of work in support of the Institute’s experimental activities. As a consequence, a large proportion of the effort has been associated with the preparation and deployment of instruments and ancillary equipment for cruise programmes.”
A major project being carried out at that time was the design and development of the ICOT Offshore Tide Gauge, including the evaluation and calibration of high accuracy low drift pressure sensors for use in this equipment.
In 1973 the Institute of Oceanographic Sciences (IOS) was formed by the merger of ICOT with the National Institute of Oceanography (NIO) at Wormley, the Unit of Coastal Sedimentation (UCS) at Taunton, and the Marine Scientific Equipment Service (MSES) at the Research Vessel Base (RVB) in Barry. David Cartwright moved from Wormley to become Assistant Director (IOS) at Bidston. Len Skinner was appointed as Head of MSES at Barry and I took over his role in the Research Technology Division at Bidston, which then became the Instrumentation and Engineering Group (IOS Bidston) with a staff of fifteen.
In 1974 Bob Spencer moved to Bidston from Wormley, where he had worked on the design and development of the NIO Offshore Tide Gauge. This work continued at Bidston with the design, development, construction and deployment of shelf edge and deep sea versions of this equipment.
With the increase in staff and equipment at that time a number of technology staff had to be accommodated in premises at the Lairage in Birkenhead where oceanographic equipment was stored, maintained and prepared for deployment at sea. The accommodation situation was considerably improved in 1975 with the completion and occupation of the Joseph Proudman Building at Bidston. As well as offices for staff, the Joseph Proudman Building had purpose designed and spacious electronics, instrumentation and calibration laboratories, a mechanical engineering design and drawing office, a well equipped mechanical engineering workshop and an assembly area for the preparation of sea-going equipment.
In the new electronics laboratory Alan Harrison and Roger Palin were joined by David Flatt and Graham Ballard in 1975. Their work concentrated on current meters, thermistor chains, CTD systems and Continental Shelf offshore tide gauges. Later work also included the measurement of flow induced voltages on submarine cables, the development and use of self-contained sea-bed mounted instrument packages (PMP), and the major design, development and use of Acoustic Doppler Current Profilers (ADCP). In the calibration laboratory Tony Banaszek specialised in the evaluation, calibration and use of high accuracy low drift pressure sensors.
Doug Leighton worked on the preparation, installation and maintenance of tide gauges at coastal sites and on offshore platforms and, with Bev Hughes, also designed, manufactured and deployed mooring systems for the deployment of oceanographic instruments at sea. Alex Kerr was responsible for the maintenance, preparation and use of the acoustic command and release systems used in mooring systems and in shallow and deep bottom mounted instrument packages.
In 1975 Bill Ainscow and Alan Browell were appointed to the Tide Gauge Inspectorate with responsibility to operate, maintain, develop and modernise the UK National Tide Gauge Network which at that time included 34 permanent tide gauges around the coast of the UK. A major development of this Network was the introduction of the remote monitoring and data transfer facilities Dataring and Dataflow, mainly designed by Roger Palin. David Smith joined the Inspectorate in 1981 and then Les Bradley in 1990, by which time 34 of the 37 stations had been modernised to include Dataring and Dataflow systems. By 1998 an improved Dataring system had been designed and the Dataflow system had been replaced by Datalink for use by the Storm Tide Warning Service and the Thames Barrier Operations Room.
In 1977 Peter Foden joined Bob Spencer in the design, construction and deployment of deep sea pressure recorders and the design, installation and maintenance of a network of island based sea level stations in the South Atlantic and Antarctic. Major developments were a deep sea bottom pressure recorder with releasable data capsules (MYRTLE), a more compact and more easily deployable deep sea bottom pressure recorder (CROCUS), deep sea bottom mounted Inverted Echo Sounders (IES), and satellite data transmission systems for data recovery from capsules and island sea level stations. In 1992 Geoff Hargreaves joined this team and then Steve Mack in 1999.
In 1985 the Taunton site of IOS was closed down and a number of Taunton staff relocated to Bidston. John Humphery moved to Bidston and continued his work on the design, development and deployment of the Sediment Transport and Boundary Layer Equipment (STABLE). He was joined by Steve Moores in 1990 and a pop-up version of this equipment was designed and built for deployment in deeper waters. In 1992 a completely new STABLE was designed and built to accommodate additional and improved sensors and with greatly increased data processing and logging capability.
Peter Hardcastle also transferred from Taunton to Bidston in 1985 and worked on instrumentation to examine the interaction of sound with suspensions. A triple-frequency Acoustic Backscatter System (ABS) was designed and used to measure sediment concentration profiles in estuarine studies. Dual-frequency self-contained instruments were designed and used for near sea-bed measurements, including a High Resolution Coherent Doppler Current Profiler (HRCDCP), a Cross Correlation Current Profiler (CCCP), and an Acoustic Bed Ripple Profiler (ABRP).
Paul Bell joined the Technology Group in 1992 and worked on the use of coastal X-band radar for oceanographic measurements. Radar reflections from waves on the sea surface are recorded and analysed using 3D fast fourier transform techniques to give directional wave number spectra. These are then used to extract the two dimensional frequency spectrum of the waves over an area. The motion of waves between successive images can also be used to yield wave velocity vectors and these can provide an estimate of the local near-shore bathymetry.
The Mechanical Design and Engineering Section was responsible for the design, manufacture and testing of all the specialised pressure housings, frames and other mechanical equipment required for the deployment of offshore instrumentation systems and equipment, and for the installation of coastal equipment. Since 1974 John Casson headed this team and he also headed the diving team required for the installation of coastal and rig tide gauges. In the engineering drawing office Judith Daniels was joined by John Mackinnon and Dave Dawson in 1976 and then by Dave Jones in 1990. The mechanical engineering workshop was headed by Kevin Taylor and other workshop staff included Alan Browell, Ken Parry, Jack Clarke, Jim McKeown and Emlyn Jones.
In 1987 IOS (Bidston) became the Proudman Oceanographic Laboratory (POL) and Brian McCartney moved from Wormley to become Director. The Instrumentation and Engineering Group then became the Technology Group with a staff of twenty-three. The scientific work of POL was then grouped into three major projects, the North Sea Project (CRP1), Dynamics of Shelf and Sea Slopes (LRP1) and Sea Level, Ocean Topography and Tides (LRP2). The Technology Group continued to support all three science projects but also had its own project, Technology Development (LRP3). LRP3 ran from 1988 to 1994 and is described as follows in the Executive Summary of the Final Report on this project:–
Proudman Oceanographic Laboratory Final Report on the Technology Development Project 1988/94 (LRP3)
Executive Summary The main objective of the POL Technology Development Project (LRP3) is to develop oceanographic observational instrumentation and equipment which will enable new, improved, more efficient or more measurements to be made more readily available to support POL scientific programmes. The principal developments are of sea-bed pressure recorders with reduced drift, increased deployment duration and in-situ data processing and recovery; sea-bed mounted acoustic doppler current profilers with potential for measuring turbulent structure in boundary layers; near-bed instrumentation to elucidate sediment erosion, transport and deposition; acoustic tomography techniques for Continental Shelf waters; and improved equipment for the modernisation of the UK permanent tide gauge network.
The principal achievements in these developments during the project have been as follows:
1. A deep sea pressure recorder with four releasable data capsules has been completed and was first deployed in 1992. The first data capsule was recovered three weeks later with excellent data. The remaining capsules will be recovered at yearly intervals and the main instrument recovered in 1996. Design, testing and construction of the satellite data link from the capsules will be completed in preparation for the first deployment of this system in 1994. The microprocessor controlled data handling and storage techniques developed have also been used in the development of island sea level stations. A new type of deep sea inverted echo sounder has been evaluated and techniques for processing and analysing the acoustic data have been developed.
2. A 1MHz self-contained sea-bed acoustic doppler current profiler has been developed using a digital signal processor, ‘C’ language software, Flash EPROM memory and more efficient acoustic transducers, to greatly improve the range, resolution, reliability, ease of use and deployment time. These techniques have also been used in the development of 250KHz and 75KHz instruments. The acoustic backscatter signal strength has been used to derive sediment concentration profiles and when combined with the current profile data provides a high resolution measurement of sediment flux. Nearly 100 deployments of these instruments have been made and new deployment techniques developed for use in high current regimes and for recovery of the ballast frame.
3. Theoretical and experimental studies on the interaction of sound with suspensions have confirmed the approach of using acoustic backscatter to make suspended sediment measurements. A triple frequency acoustic backscatter system has been designed and used to measure sediment concentration profiles in estuarine studies. Dual frequency self-contained instruments have been designed and used for measurements at sea. A prototype coherent doppler system for high resolution current profile measurements near the sea bed has been designed and tested. The sediment transport and boundary layer equipment has been completely redesigned with high capacity data loggers and the first deployment was successfully completed in February 1993. For the first time this provided a complete data set describing the benthic current and pressure environment and the associated suspended sediment profiles.
4. Modernisation of the UK permanent tide gauge network has been completed at 24 sites, and a further four will be completed by March 1994. High speed modems are being introduced to improve data transfer, and a new workstation has been installed at POL to control the network and to improve data processing, presentation and evaluation. A three site network has been designed and installed at Barrow-in-Furness, including an offshore site. A real time system has been installed at five east coast sites providing data directly to the STWS. A mid-tide pressure sensor system has been designed, evaluated and is being installed at new sites to improve datum control.
5. An extensive review of acoustic tomography has been completed and a report written presenting the theoretical background and highlighting the majority of experiments which have been conducted, mainly in the deep oceans. Shallow waters are acoustically more complex and POL interests would probably have to concentrate on relatively simple applications of the technique.
In the early nineties the North Sea project (CRP1) was followed by a new Community Research Project called Land-Ocean Interaction Study (LOIS) with two main strands at POL, Ocean-Shelf Interactions (SES) and Coastal and Shelf Interactions (RACS). The Technology Group continued to support all the scientific projects at POL with the deployment and use of existing instrumentation systems and the development and use of new and improved systems.
In 1994 the Centre for Coastal and Marine Sciences (CCMS) was formed by the merger of POL with the Plymouth Marine Laboratory (PML) and the Dunstaffnage Marine Laboratory (DML). This did result in some technology collaboration between the three Laboratories and a CCMS Technology Development Project was proposed in 1997. However, before this was fully implemented CCMS was disbanded in 2000. POL moved from Bidston to the University of Liverpool in 2004 and became part of the National Oceanography Centre (NOC) in 2010.
After the completion of LRP3 in 1994 further technology work at POL included the development of a small, freely drifting, neutrally buoyant oceanographic buoy with the capability of undulating throughout the water column. The buoyancy is adjusted under microprocessor control to enable the buoy to sink or rise at a desired rate, to drift at a set depth, to sit on the sea bed, or to drift on the surface. At the surface the position of the buoy would be determined using a GPS receiver and two-way communication would be established by VHF or satellite link to transfer recorded data and to reprogram the buoy. Pressure, temperature and conductivity can be recorded and other sensors added as required.
Another development, in collaboration with IESSG at the University of Nottingham, was of a moored surface following buoy incorporating a dual-frequency GPS receiver. GPS data and measurements of the inclination and freeboard of the buoy are transmitted by VHF radio to a shore based reference station. Data from a GPS receiver at the reference station can be combined with the buoy data to calculate the sea-surface level at the buoy relative to the reference station level every 2 seconds, to an accuracy of about 3cm. Offshore sea-level, surges, tides and waves can then be computed at the reference station and transmitted by telecom link in near real time.
To see further details and illustrations of some of the development projects carried out by the Bidston Technology Group please click the thumbnails below.
Shortly before retiring from Bidston Observatory in 1999 I put together two documents, one of these is a compilation of all the Bidston Technology Group Annual Reports from 1969 to 1998, and the other a compilation of all the minutes of the POL Fieldwork and Scientific Support Committee between 1986 and 1999. The former document includes a copy of the Final Report of the Technology Development Project 1988/94 (LRP3), referred to earlier, and the front page lists all of the staff in the Technology Group between 1969 and 1999, fifty in total, with the numbers increasing from nine in 1970 to a maximum of twenty-seven in 1995. Anyone interested in seeing these documents should inquire with the National Oceanographic Library at the National Oceanography Centre in Southampton.
On retiring I was delighted to be presented with the very appropriate gift of a walking GPS receiver from the POL staff. As well as accurately measuring and recording latitude and longitude anywhere on the surface of the Earth this remarkable little instrument uses an atmospheric pressure sensor, calibrated with the GPS signal, to also measure and record its elevation above sea level. I can now report that since then I have made very good use of this instrument, although recently superseded by an even more remarkable smartphone, in navigating my way over the hills and mountains of Snowdonia and the Lake District.
This is the text of a speech given by Sylvia Asquith on 27th September 2017 at the Foundation of Art and Creative Technology (FACT) during the New Observatory Exhibition. Sylvia’s speech was followed by the screening of a short film by Yu-Chen Wang entitled “I wish to communicate with you”.
Good evening ladies and gentlemen.
My name is Sylvia Asquith and I joined the Bidston Observatory staff in February 1947 as Sylvia Brooks. It was a long time ago but I well remember those early days.
I was employed as a junior member of staff comprising six women and two men – Dr Doodson and Dr Corkan. As well as learning to be a meteorological observer I was introduced to two ammazing tide predicting machines! These were kept running continuously from 9 a.m. to 6.30 p.m. (4.30 to 6.30 being overtime) at 2 shillings and 6 pence per hour which is 12½ pence per hour in current money, except that we do not even have ½pence any more. The minimum wage obviously did not exist in those days!
During the wartime years the male members of staff were enlisted in the armed forces while the women gallantly went on with the work which was so vital at that time. They also did fire-watching on the roof and could tackle incendiary bombs very efficiently.
The Roberts-Légé machine was moved to a purpose-built underground room in the grounds and kept running from there as protection in case the Observatory was bombed. The Kelvin machine was already kept in a cellar. There were incidents of bombs falling on the roof and on the hill generally and on one occasion a landmine landed near the building causing windows to be blown out but without causing significant damage.
After the war Dr Rossiter and two female staff were demobbed and returned to their duties at the Observatory.
After joining in 1947, I received instruction on running a tidal machine, stopping at the correct moment and reading off the time showing at the zero point, and noting down high and low waters in succession. Times done first and the high and low to correspond. Also, you need to check the data when taking over from someone else in case they had got it a day out. All the wheels and pulleys are connected by a fine gold wire and represent forces of the moon and sun on the tide. As the biggest influence on the tide is the moon, that is represented by the largest wheel, “M2”, which has the largest amplitude on it. The names denote George Darwin, a relation of Charles, who was first to devise the method of harmonic analysis. The “M” and the letters nearest to it alphabetically refer to the Moon and similarly “S” to the Sun. The “2” means twice, etc.
The machines were stripped down and cleaned regularly by the three Doctors. However, one day they were away on business in London and the belt snapped! Someone remembered that bootlaces sewn together and carefully measured would be a good standby, so a staff member rushed to the shops and came back with laces and with a sewing machine supplied by Mrs Doodson from the house, and a new belt was made. Fitting was a bit tricky but we managed and the work was able to continue. The returning staff were most impressed by the ingenuity of their colleagues.
Returning to the process… The predictions came off the machine and the sequences were differenced and the differences smoothed by a senior staff member. The smoothed predictions were then typed up for publication in the Admiralty Tide Tables and photograph copies made of the originals. Yes, we even had a photographic studio and print room on site. We always tried to work two years in advance to allow for the checking and printing of predictions.
For all tidal predictions a tidal analysis is required using twelve months of actual height values and following the completion of this any year can be put onto the machine going as far back or as far forward as desired. That means that, provided we know the exact date in history for an event, we could identify the tidal conditions existing at that time.
Bidston was also a Met Office recording station and I was put in charge of the observers. I’m proud to say that we received two awards for our Met returns to London, indicating the very high standards, consistency and quality of our recording and reporting.
The one o’clock gun was a feature of the Observatory dating back to Victorian times giving an absolute and accurate timing to enable chronometers for shipping and people and businesses across Merseyside to set their own timepieces by. This was resumed in 1946 and fired electronically every day from Bidston to the gun at Morpeth Dock. Eventually this finished on July 18th 1969 and I had the pleasure of being the last person to fire the gun.
I returned part time in 1967 after ten years working from home – not so much of a modern concept as you may have thought – and continued as a Scientific Officer until retirement in 1990. Yes, I saw many changes in my 43 years association with Bidston, from a staff of 8 to a staff of 80 housed in two buildings. Today computerisation means that predictions now take microseconds where at the start using these machines they took about three days per port, but we are talking pre-computer times and these machines represented the height of technology in their own era and as such deserve their place in history.
I hope that I have given you a flavour of the history and use of these machines and the fantastic team that I worked with to operate them.
We should always remember that the outputs from these machines were used by individuals and organisations across the world who depended totally on their accuracy to help ensure safey at sea and around coastlines.
Thank you for listening and I hope that you enjoy the film.
The connection between storm surges in the North Sea and the new British Nobel Laureate, Kazuo Ishiguro
Judith Wolf, October 2017
I only met Kazuo Ishiguro’s father once. In April 1981 we both attended a session of the 5th UK Geophysical Assembly at the University of Cambridge. I was in the throes of my PhD study and looking at the effect of wind gustiness on wind-driven currents in numerical models. In our session, on “Air-Sea Interaction” there were only three of us (the third being Ed Monahan, who worked on wind waves), and being the last session on the Friday afternoon, and rather peripheral to the main topics of the conference, there were only the three of us left there to listen to each other’s presentations and dutifully ask questions. Shizuo Ishiguro’s talk was entitled “Extreme surge predictions by the quasi uniform steady wind/pressure field method” (*); he was known to me by reputation, although by this time his work was something of an anachronism, as the world had moved on to digital computers. He had built an analogue computer to model North Sea storm surges and was employed, like myself, at the Institute of Oceanographic Sciences (IOS), but based at Wormley in Surrey, while I worked at Bidston Observatory in NW England.
His storm surge calculator was an electronic analogue computer, originally developed at the Marine Observatory in Nagasaki, Japan for applications in the fishing industry. In 1957 he came to England, at the invitation of George Deacon, the Director of National Institute of Oceanography (the precursor of IOS Wormley) in Wormley after they met at a tsunami conference in Japan. Initially he was on a UNESCO fellowship, then joined the permanent staff in 1959 as an “oceanography engineer” and developed his machine to model North Sea storm surges. The calculator converted hydrographic and meteorological data into voltages and electric currents and produced output on an oscilloscope relating to the behaviour of the surge wave at coastal locations.
When he retired he took the calculating machine home and tinkered with it in his shed until his death in 2007. It is now in the mathematical collection, the Winton Gallery, in the Science Museum in London, which I visited on 1 January 2017, seeing the machine at first hand, for the first time.
Shizuo’s son, Kazuo Ishiguro, was born in Nagasaki in 1954, but moved with his family to the UK, as a boy. He has said that if he hadn’t moved to the UK he might never have become a writer.
Operation Weather Rescue needs volunteers to help digitise 2 million meteorological observations taken at the weather observatory on top of Ben Nevis between 1893 and 1904. These data remain the most comprehensive set of mountain weather observations ever taken in the UK and could help answer many scientific questions.
This short film, by Andy Lane, Andy Heath and Craig Corbett, is part of the Tide and Time exhibition at the National Oceanography Centre, Liverpool. The exhibition showcases two tidal prediction machines – the Roberts-Légé and the Doodson-Légé. The film also explores the history of tidal science in Liverpool and its development as a port.
There is a lot of renewed interest in tide prediction machines and, after many years hidden away in storerooms, some of the machines made in the UK are on permanent display once again. Kelvin’s original 10-component machine is now part of the new Winton Gallery for Mathematics at the Science Museum in London alongside Ishiguro’s storm surge simulator. Two of the machines that were used at Bidston can now be seen at the National Oceanography Centre building in Brownlow Street in Liverpool.
As you may know from articles mentioned in the Resources section of this web site, the tide prediction machines were a way of simulating the tide in terms of its many harmonic components. Each component would be represented by an amplitude and phase lag, called the ‘harmonic constants’, and the machine, which can be considered as a sort of analogue computer, would be programmed to run by providing it with these constants. Of course, the constants would differ from port to port.
That raises the obvious question of where people like Arthur Doodson, and the other operators of the machines, got their constants from in the first place. This short article reviews the main characteristics of one of the machines (the so-called Doodson-Légé machine now on display at NOC) and then attempts to answer the question of how Doodson obtained the constants.
The Doodson-Légé Machine
The Doodson-Légé machine simulates the variations of the ocean tide by representing the tide as a combination of 42 constituents, each of which has a particular amplitude (h) and phase lag (g). The values of h and g are ‘programmed’ into the settings of the 42 wheels, and the nickel tape that wraps around and connects all the wheels serves to sum up all the constituents.
What are these constituents? Mathematically, the total tide htotal at time t can be expressed as a sum of many cosine series (one for each constituent). We can write this schematically as:
htotal(t) = ∑i=1,…,42hicos(ωit − gi)
where hi, gi and ωi are the amplitude, phase and angular speed of constituent i, and ωi = 2π / Ti with Ti its period. The periods of the 42 constituents correspond to the known main lunar and solar frequencies which contribute to the tide. Most of them have values around either 12 or 24 hours (semi-diurnal and diurnal tides), some have smaller values (shallow-water tides) and a few have values up to a year (the long period tides). The two largest constituents in many parts of the world, including Liverpool, are called:
M2, with a period of 12 h 24 min (the main semi-diurnal tide from the Moon with a period of half a lunar day) and
S2, with a period of 12 h (the main semi-diurnal tide from the Sun with a period of half a solar day).
At Liverpool, M2 and S2 have amplitudes of 3.13 and 1.01 m respectively. Because our day is 24 hours, the S2 tide will repeat itself twice a day exactly at the same time every day (shown in red below). M2 has a larger amplitude and repeats twice a lunar day (a little later each time) as shown in blue. They combine by ‘beating together’ to give a classic ‘semi-diurnal’ tide where M2 and S2 together result in a total tide that is larger and smaller over a fortnight, called spring and neap tides.
You can appreciate that simply by combining the separate contributions of these two constituents (M2 and S2), we already have a curve which starts to look something like the real tidal variation at Liverpool over a fortnight.
The fact that the tide can be parameterised this way, as a simple addition of harmonics (but many more than two), made it technically easy to invent machines such as the D-L machine that could sum them up. Some machines could handle 40 or more constituents. That was important in order to be able to handle the many smaller constituents that contribute to the tide (not just M2 and S2). Also, in other parts of the world, the total tide can have very different characteristics to that at Liverpool and so the machines needed to be able to handle their particular constituents. See Pugh and Woodworth (2014) for a discussion of why these different types of tide occur.
Obtaining the Harmonic Constants to give to the D-L Machine
As mentioned, the D-L machine has 42 wheels (or constituents), which means that we need 84 numbers to ‘programme’ it (i.e. the amplitude and phase for each constituent for the port in question). Once it has been set up correctly, then it can be run to predict the tide at the port for many years in the future (or past).
But how did Doodson know what these 84 numbers were in the first place?
The 84 numbers come from analysis of previous observations of the tide at the port using a tide gauge. Normally one year of data was adequate, with observations of the water level every hour (i.e. about 9000 values in a year). The team of people who worked with Doodson (called his ‘computers’) usually worked with values of water level in units of 1/10 of a foot and expressed as integers. There is a lot of arithmetic involved in this work and integers are much easier to deal with than real numbers.
His method of analysis of the hourly values made use of ingenious arithmetical filters designed to emphasise the importance of each constituent in turn and so, after a lot of work, arrive a set of estimates of amplitude and phase for all 42 that could be used to programme the machine. The work was very labour intensive, involving endless integer arithmetic by someone who could add up properly. Analysis of a year of data could take a ‘computer’ a few days or a week. The procedures are described in Doodson (1928) and Doodson and Warburg (1941) although be warned that a reader has to devote some hours to understand them.
Now, the people in those days before digital computers could not readily handle 9000 hourly water level values in most of their work, and it turns out that for Doodson’s tidal analysis it is not necessary, as long as the data is good quality. Doodson invented a set of filters which would convert the hourly information into daily numbers, which are just as useful for the tidal analysis, and have 24 times less the bulk of the original data.
For the semi-diurnal constituents these filters are called X2 and Y2 and are a set of simple integer arithmetic weights applied to the hourly values for each day. (They can be thought of as representing the real and imaginary parts of the variations. For people used to studying satellite altimeter data from space the outputs of the filters are akin to the aliasing that occurs in tidal lines using ‘repeat track’ data.)
The method they used was to list the hourly values from hours 0 to 23 each day on a page, one line for each day, and then have a cardboard cut-out with holes for the hours which had to be multiplied by a filter weight. These cut-outs were called stencils. The weight value itself was written on the cardboard, black for a positive weight and red for a negative weight.
The X2 filter for a particular day used data for hours 0-23 on that day and also hours 24 to 28 (i.e. hours 0-4 on the next day), spanning 29 hours total. The integer weights were:
Note that the central value is 3 hours different from that of X2. So the two filters sampled orthogonal (or real and imaginary) components of the semi-diurnal variation.
Nowadays, we can easily apply these filters to our example Liverpool data using a computer and the daily record of X2 and Y2 time series for Liverpool then look something like:
You can see it has much the same information content as Figure 3 (i.e. variation over a fortnight and with two sets of amplitudes and phases) but with 24 times fewer numbers. The constant parts (the offsets) of the red and blue curves come from S2, because S2 is the same every day. And the cyclic parts come from M2, which varies over a fortnight. The red and blue offsets give the amplitude and phase of S2, and the amplitudes and phases of the cyclic parts give the amplitude and phase of M2.
So the first task of the ‘computer’ person was to calculate X2 and Y2 for each day (the work was done by hand of course, or sometimes with the novelty of an adding machine) and write the values for each day in a table with 12 columns (for 12 months of the year), with some columns having 29 rows, and some 30 rows. X2 (or Y2) values for days from the start to the end of the year (i.e. about 360 values) would be listed down column 1 first, then down column 2 etc. until the year was completed in column 12.
The second step is harder to describe but in fact is the most important. The X2 (or Y2) values in each of the columns of the 12 months in the first table were multiplied by different sets of integer weights to maximise the importance of the many different constituents with slightly different frequencies (called ‘daily multipliers’, see Table XV of Doodson, 1928). In addition, sums of the X2 (or Y2) values in each month, listed in each column of the first table, were multiplied by further several sets of integer weights for each month (called ‘monthly multipliers’, see Table XVI of Doodson, 1928). In our idealised example, after a lot of arithmetic, that readily leads to 4 numbers i.e. the 2 each we need for M2 and S2.
The different constituents with periods around 12 hours (other than M2 and S2) have names like 2N2, Mu2, N2, L2, K2 etc. (and it will be seen that they all have their own wheels on the machine). They will all contribute to the hourly water levels to make the total tide plot of hourly values more complicated (Figure 3), and also to contribute individually to the X2 and Y2 values (Figure 6). The results of the filtering in the second step, by means of the daily and monthly multipliers, produces values that have contributions in different amounts from each semi-diurnal constituent. Therefore, the information from the second step needs to be recombined, using linear combinations of each parameter in a process that Doodson called ‘Correction’, in order to provide information specific to each constituent. This was also labour intensive but it was straightforward once a clearly-defined procedure could be explained to a ‘computer’. The method is described in great detail in his 1928 paper with a worked example for Vancouver (which has some mistakes that do not matter).
Doodson then had all the amplitudes and phases that he needed to programme the machine. (In fact, the long-period tides were treated differently but they are not important for this note.) All the amplitudes and phases were written down on a special ‘constants card’ for the port in question which the machine operator would use whenever the machine was required for setting up for that port and year in the future; Figure 7a,b shows the front and reverse of an example card.
The top part of the first card shows amplitudes for each constituent in the order they appear on the machine for Hilbre Island for 1987 and 1988. The amplitudes are in metres and the lunar ones are slightly different each year because of the nodal variations (the ‘f’ factors). You can see the amplitudes for the solar constituents such as S2 are the same both years. The lower part of the card shows the values of frequency * amplitude for each year, with an overall scaling factor, which represents the rate of change of the constituent.
The amplitudes are programmed onto the shafts (also called ‘amplitude blocks’) for each wheel on the front of the machine using a Vernier screw, and the values of frequency * amplitude are programed similarly on the back of the machine (i.e. the machine is a ‘double sided’ one, in effect two separate machines, one for the heights and one for the rates). Finally the phases for each constituent, shown on the reverse of the card for each year (e.g. Figure 7b), are programmed onto the wheels at the front of the machine using the rotating dials. In order to rotate the dials, it is first necessary to release the associated clutch, remembering to tighten it up again before running the machine, otherwise the contribution of the particular constituent would not be included in the total tide. These phases are not phase lags (or they would be the same for both years) but are values of V+u-g, where ‘V’ is the astronomical argument for the start of the year, ‘u’ is the nodal correction, and ‘g’ is the phase lag. These can all be readily computed for each year once one knows ‘h’ and ‘g’, as explained above.
Was this Method of Doodson the Best or Easiest for Finding the Amplitudes and Phases?
The Doodson method described above was by no means the first. You can find many papers from the 19th century which have lists of amplitudes and phases for the various constituents computed in different ways (e.g. see Baird and Darwin, 1885).
For example, in the 19th century there had been methods:
The British Association method, as used by Kelvin, Roberts and Darwin. This was the method used by Roberts to determine amplitudes and phases with which to programme his earlier tide prediction machines.
Darwin’s own later method.
A method used by the US Coast and Geodetic Survey.
Börgen’s method in Germany.
The methods differed in the amount of labour involved, in how well they could eliminate the overlap of information from different constituents in the derivation of the amplitudes and phases, and in the completeness of the analysis. The BA method was the most arduous for the ‘computer’ person.
Doodson’s method was largely an extension, and more complete version, of that employed by Darwin many years before. As always with Doodson, it was devised with more than one eye on subsequent application of the tidal information for use by the prediction machines. Doodson was excellent at handling numbers (and, as important, in showing how his ‘computers’ could handle the numbers) and the method he devised, although complicated and long-winded, was perfectly adapted to routine working by people with basic mathematical skills.
Finally, we can refer to the work of Lord Kelvin (William Thomson), who not only invented the ‘Kelvin Machines’, as the Tide Prediction Machines (TPMs) like the Doodson machine were called, but also invented a mechanical analyser which he thought should be capable of the numerical tidal analysis described above. His prototype ‘tidal analyser’ allowed for determining 5 constituents and a later one allowed for 11. The 5 constituent machine can be seen at Glasgow University, while the 11 constituent machine is in the Science Museum, and both are described by Hughes (2005).
However, Kelvin’s analysers were not successful and so hand-calculated computations of the Darwin and Doodson type were needed for many years. Nowadays, all the tidal analysis of a year of data can be performed in a split second on a modern computer using methods that have many similarities to those of Doodson (see Pugh and Woodworth, 2014).
Valerie Doodson and Ian Vassie are thanked for comments on a first draft of this article.
Baird, A.W. and Darwin, G.H. 1885. Results of the harmonic analysis of tidal observations. Philosophical Transactions of the Royal Society, 34, 135-207.
Doodson, A.T. 1928. The analysis of tidal observations. Philosophical Transactions of the Royal Society, A 227, 223-279.
Doodson, A.T. 1951. New tide-prediction machines. International Hydrographic Review, 28(2), 88-91 and 6 plates.
Doodson, A.T. and Warburg, H.D. 1941. Admiralty Manual of Tides, His Majesty’s Stationery Office, 270pp.
Hughes, P. 2005. A study in the development of primitive and modern tide tables. PhD Thesis, Liverpool John Moores University.
Pugh, D.T. and Woodworth, P.L. 2014. Sea-level science: Understanding tides, surges, tsunamis and mean sea-level changes. Cambridge: Cambridge University Press. ISBN 9781107028197. 408pp.
The exhibition – at the National Oceanography Centre in Liverpool – showcases some of the fascinating achievements made in the Liverpool area in understanding and predicting the tides. The highlights of the exhibition are the rare Roberts-Légé and Doodson-Légé tide prediction machines, extraordinary analogue computers that calculate the rise and fall of the ocean tide. See these beautifully intricate machines up and running at the only place in the world where you can see two of them together.
Bidston Observatory was the home of the Roberts-Légé and Doodson-Légé tide prediction machines while they were still in use. The machines are now owned by National Museums Liverpool, who have carefully restored them to working condition.
Tide & Time is open to the public once a month (usually the first Tuesday of each month from 15:00 to 16:00) or by special arrangement for group visits and events. See this page for information on planning your visit and how to book.
The exhibition will also be open to the public during LightNight Liverpool on Friday 19th May 2017 from 17:00 to 22:00.
Originally, from 1955, I worked in the Met Office at Speke Airport (later to be called Liverpool Airport and subsequently John Lennon Airport). I very much enjoyed being a weather observer – sending observations up to the control tower to be passed on to aircraft, but the job involved shift work, which included regular night duties. This was fine till I got married in 1961. At that stage, I became less enthusiastic about shift work and about the amount of travelling involved between Greasby and the airport: bus – ferry – bus – at least an hour each way. I didn’t drive in those days.
So I decided to look for another job. Bidston Observatory came to mind. It was much nearer home and I knew they had a weather station there. So I wrote to the Director asking him if there were any job vacancies. He – Dr. Rossiter – invited me to go for interview and duly offered me a job! It was as easy as that in 1961. Nowadays, with high competition for every post, people can’t believe that it could ever be that easy.
I was a very basic assistant at Bidston – one of 10 girls who were classed as ‘computers’. We operated tidal prediction machines – large machines consisting of gears, weights and pulleys which could be set to represent the contributions of sun, moon, location, etc. to the tides of a port. You can read all about these machines in other articles on this site.
The scientific programs which turned these numbers into tidal predictions were written by the scientists – them upstairs! – it was all way beyond our understanding. We just operated the machines by foot pedals and a hand wheel and wrote down the answers – the more senior girls scanned our numbers looking for obvious errors. When plotted on a graph, the figures would form a smooth curve representing the pattern of the tide on consecutive days at the port concerned. Once the figures had been accepted as correct, we had to write them down on prepared forms – using pen and ink – no biros allowed – neat handwriting was essential for the job! There was a darkroom in the basement where our carefully written-out tables were photographed before being sent to the port authority concerned. This was a typically old-fashioned dark room with trays of chemical developers, subdued red lights, etc. In those days we did tidal predictions for many parts of the Commonwealth.
Another of the girls’ duties was to maintain a daily weather diary. At 9 am each day – Saturdays, Sundays and Christmas Day included – the duty observer would take readings from the thermometers in the Stevenson’s Met. Screen sited on the Observatory lawn, change the temperature and humidity charts on the analogue instruments also sited in the met screen and change the chart in the tipping bucket rain gauge, as well as measuring any rainfall recorded in the rain bottle. The observer would then go up to the roof to change the daily sunshine card in the Campbell-Stokes sunshine recorder. The sun’s rays were concentrated through a solid glass ball to produce a burn on the specially-treated card. In the summer, this recorder was located on the roof of the ‘Dines cabin’ – the climb up the ladder to this site could be rather precarious on a windy day. In winter, the sunshine recorder was moved to the outside of one of the domes accessed from inside the dome by a small door (again up steps) facing due south. Because the sun is a lot lower in the sky in winter, and needing a smaller range of exposure, this was obviously safer for the staff than the outside summer climb.
Inside the ‘Dines cabin’ was the Dines anemometer recording wind speed and direction on an analogue chart. There again the observer changed the chart on the instrument’s cylinder. The final job was to note the visibility from all sides of the roof. On fine days, we had a great view over Liverpool with the Pennines in the distance. To the north, we could see Blackpool and occasionally Black Coombe in Southern Scotland. To the west, we could see the Great Orme and the Snowdonia range.
Taking the retrieved charts and the sunshine card, the observer returned to the office and calculated three hour readings for the past 24 hours and entered them into the weather diary. These diaries were beautifully produced for us by a company in Liverpool and, I believe, they are now housed in the Wirral Libraries Archive in the Cheshire Lines Building in Birkenhead.
Another job for the duty observer was to fire the one o’clock gun at precisely 1 pm Mondays to Fridays. This was a tradition dating back to the building of the Observatory in 1866, when accurate time was not available to the business people of Liverpool. A very accurate clock in the Observatory was connected by landline to a gun sited at Morpeth Dock, on the Birkenhead side of the Mersey. When the observer flicked a switch at Bidston the gunfire was heard in Liverpool (the gun having first been duly primed by a docker at Morpeth). The practice was discontinued at Bidston in 1969, but still continues at observatories in other parts of the world.
The girls had little association with the scientists who were mostly men. At coffee time – strictly 1045-1100 am (we daren’t overstay our time limit) – the men stood round the marble fireplace in the old dining room and the girls sat at the tables. There was little communication between the two groups. Incidentally, the girls prepared the coffee on a rota bases – strictly 50% warm milk – heated in a pan and 50% water. When the coffee was ready, spot on 1045 am, we pressed a buzzer – I think it was 2 buzzes for coffee break – to summon the staff from upstairs.
At lunch time, on a fine day, the menfolk would often take a brisk walk over Bidston Hill usually talking shop. The girls tended to sit on the observatory front door step eating their sandwiches.
It was quite a hierarchical situation at the observatory in those days – a total staff of only about 18 people – a sort of strict family atmosphere – and always quiet. I enjoyed working there.
When I was expecting my first baby in 1964, people seemed quite relieved. It was several years since anyone had become a mum and they had thought there was a hoodoo on the place! Dr. Rossiter was very solicitous towards me when I became pregnant – he insisted on my desk being moved downstairs to save me having to climb anywhere or do anything at all strenuous. There was no thought of my returning to work after having the baby. Mums did not return to work in those days! In the event, I did return to Bidston part time when my younger son was nine years old and attitudes towards working mums were starting to ease.
More stories of life at Bidston Observatory at this time can be found in my book “Bidston Observatory: The Place and the People” (Countryvise Ltd. 2006. ISBN: 978190121687).
I was recently invited to attend a garden party to celebrate 150 years of the Bidston Observatory, hosted by Stephen and Mandy Pickles on Saturday 17 September 2016 in the grounds of Bidston Lighthouse. This gave me a deep sense of déjà vu, as it reminded me so much of my first day as a member of Bidston staff at the start of 1972.
On that day, I drove up the same well-worn drive, past the sandstone wall entrance, and into the grounds. On my right hand side was a lawn that was shortly to be occupied by the new Proudman Building. But in early 1972 that area looked almost the same as it does now, except for a small vegetable patch that was attended to by a Mr. Connell. He and his family occupied the cottages that belonged to the lighthouse and had been built by the Mersey Docks and Harbour Board. On that balmy Saturday evening in September, I thought it quite strange that, here I was celebrating 150 years of the Observatory, and yet the ‘new’ Proudman Building had been built and demolished (in early 2013) within little more than 40 years, a fraction of the Observatory’s lifetime.
The nostalgia continued as I parked my car behind the rear of the Observatory in almost the same spot as I had on that first day at work. I remembered thinking back; my father would quite often force me to join him on one of his marathon walks. One of his favorite treks was from Moreton to Bidston, then over the Vyner Road footbridge, past the windmill, around the Observatory boundary wall down to the village, then home. In the 1950s and early 60s, I was infatuated by science fiction and men-from-outer-space movies, and TV dramas like Quatermass and Doomwatch. For me, looking over the walls surrounding the Observatory presented all kinds of mysteries: What secrets were hidden inside the huge white domes? My youthful and vivid imagination had no bounds in ‘them days’.
On my first day in 1972, I now had the chance to look at the Observatory from the inside out, as opposed to the outside in. How exciting! As I got out of my car and approached the entrance, a gentleman in front of me held the door open and greeted me with the words “Hello Kevin, glad to see you are joining us”. We then passed through the vestibule door and continued to chat in the hallway for a good ten minutes. He then finished by saying “you will be with Dr. Skinner’s group. I will take you to his office”. He gave a quick knock on the door, popped his head around, and said “Sorry Len, Kevin is not late, my fault I kept him chatting”. I was later taken through to the rear of the building for the mid-morning tea break when the same gentleman entered. I turned to one of the staff and asked “who the nice man was”. “That is Dr. Rossiter our director” was the reply. I was then informed that he was the brother of that brilliant actor Leonard Rossiter from the Rising Damp and Reginald Perrin television shows (come to think of it, they did look alike). [Editor’s Note: see mention of Rossiter and other Bidston Directors in an article by Graham Alcock].
So, allow me to digress about a couple of things that have struck me about time, and why I have given the title of this article as ‘Reflections on Time.’ It seems to me that we have different perceptions of time depending on the situation. For example, my first day at the Observatory was over forty years ago, and yet on that recent Saturday in September, it felt like only yesterday. Another example concerns my grandmother, who was 104 years of age when she passed away. When she was born in 1889, the Observatory building had been completed (in 1866) only 23 years before. So, why were we so concerned with celebrating the Observatory as an ‘historic building’, when my memories of my grandmother do not feel ‘historic’? She was just my Nan. So, time is a funny business.
One of the main reasons for the Observatory was to provide accurate time. This gives me a chance to refer to a hero of mine called John Harrison, who had nothing to do directly with the Observatory but, of course, also had an important role in our maritime history. When a fleet of warships ran aground with the loss of many lives and ships due to bad navigation, a vast reward was offered by the King to anybody who could come up with a good way to improve navigation at sea. The main problem was how to calculate longitude, and many ideas were offered: for example, a crazy scheme for anchoring old redundant ships at fixed positions apart, distributed across the whole ocean. The establishment was convinced that the only way that longitude could be calculated, was by using the stars and planets. Harrison in the meantime concentrated on trying to develop a precision marine chronometer. His theory, that longitude could be calculated by the use of time to good precision, was treated with great disdain.
To prove his theory, he would be entirely dependent on producing an accurate timepiece. This proved to be a formidable task. Not only had it to overcome a ship’s movement, but temperature played a significant part in the reliability of the timepieces he produced. Originally, clocks used a pendulum and weight with an escapement movement, but temperature would increase and decrease the length of the pendulum, making the precision he was looking for unsatisfactory. He spent many years trying to overcome this, by making the pendulum out of metal rods with different thermal coefficients of expansion, but alas to no avail. It was not until the latter part of his life that he produced the famous Harrison timepiece. The connection to the Observatory in this story is, of course, that the calibration of marine chronometers was subsequently to form an important part of activities at Bidston, in addition to the astronomical work in establishing the longitude of the port of Liverpool.
Accurate time has historically not been very important for most ordinary people – the sun came up, the sun went down, and what happened in between was neither here nor there. However, for those people who did need accurate timing (on land), the development of affordable watches and clocks, supplemented by sundials, was enabling decent and routine measurements of time by the end of the 18th century. One way of providing accurate timing information to the general population was by the use of time balls controlled by nearby observatories such as Bidston. A time ball was a large sphere (a ball) on top of a shaft positioned on the roof of a prominent building. At precisely midday (or another time such as 1 pm), the sphere would be dropped and people (including ships’ captains) would set their watches. This was a satisfactory situation only when visibility due to the weather allowed the time ball to be seen. Instead, the time balls were eventually complemented by an audible signal such as made by a canon. Hence, the famous Liverpool “One O’clock Gun” came into being. Originally the Liverpool Observatory was located at Waterloo Dock, and the gun (a remnant of the Crimean War) was fired from the Liverpool side of the Mersey. An improvement was made by moving the Observatory from Liverpool to the highest point on the Wirral side of the river, but close to the Dock Estate, this being Bidston Hill. The gun was relocated to Morpeth Dock in Birkenhead, and was now fired directly by an electrical signal from the Observatory.
Time eventually became a significant factor in everyone’s life, and now controls our lives more and more. Everyone knows about the advent of the industrial revolution, and the development of the railway, and the national adoption of Greenwich Mean Time. Now we are controlled by our smart-phones by time that comes from space via GPS satellites. Everyone is in a hurry or we’ll be ‘late’.
So I have been thinking back to that first day at work. At that time, I had many questions, such as “Why is the Observatory called The Institute of Coastal Oceanography and Tides, or ICOT for short?” Or, “What has oceanography got to do with astronomical observations?” These questions were answered for me over the years as I got to understand the relationships between the heavens and earth, and in particular the relationships between time and the tides, and so the ocean, and how these topics have evolved to become a crucial part of everyday life.
This has been a very brief look, from my perspective, at ‘time’ and at some small aspects of life at Bidston Observatory. It would take many volumes to do it justice to it regarding topics such as the development of tide tables, the use of precise instruments (e.g. for earth tides), the collection of oceanographic data from around the world, the fieldwork at many locations etc. Perhaps other people can cover these topics on this web site. Some of the world’s most famous oceanographic scientists have worked at or passed through the Observatory during its history. I feel very fortunate to have experienced a small part of the wealth of that Bidston history. And I hope that its historical significance is appreciated by future generations.