History of the Cromwell Tower Observatory

The Cromwell Tower has only been called so since about 18801. The name refers to the time of its origin, in the 1650s. In 1651, Cromwell's army marched upon Aberdeen and the city fathers had the wit to realise that resistance was futile, thus sparing themselves much of the destruction and pillage that other towns had experienced. King's College, a residential University with Papal foundation, was a conspicuous target for reform and, not surprisingly, the Principal and Sub-Principal were replaced by puritans.

The Cromwell Tower can be seen at top right, during building in 1661

Far from this being the death of the College, there followed an increased student roll and, in response to an accommodation shortage, the 'Cromwell Tower' was erected over the 4 years 1658 - 1662. The 'tower-house' architecture was in keeping with that found in Scottish castles, a connection that is probably no accident. The 'New Building', as it was simply called, was 14.6 m by 11 m by 21 m high, with walls 1.5 m (5 feet) thick. It had 24 rooms, chimneys and 'conveniences', mainly intended as residential accommodation. Money was raised to pay for the building by appeal to College staff, to officers of the occupying Cromwellian army (who gave generously) and to the well-connected gentry and professionals of the area. Those with a good historical memory will realise that the building would have been incomplete in 1660 when Charles II returned to the throne after Cromwell's death. "The episcopal clergy helped to complete the work begun by their opponents", as one historian wrote, and the Exchequer found the remaining funds needed to finish the building. The ornate turret on the top visible in the 1693 print was blown down in 1715 and, in spite of what one University historian says, it must have been rebuilt because later pictures show it.

Centre right - the completed tower in 1693


The tower's main use for the next 150 year seems to have been as accommodation. Was the observatory part of the original structure? It may look like it from the accompanying pictures but the ornate tower was not officially classed as an observatory and is not the structure we see today. The observatory of today was built in 1826, give or take a year. It was a last minute addition from Alexander Dauney, Professor of Civil Law, to a substantial programme of repairs and re-furbishment to the College that had been discussed since 1822. The programme was held up because of the perennial problem of matching costs to funds available. Matters came to a head in 1825 and at that point Dauney proposed significant changes, including the observatory. Dauney was a keen astronomer and possessed a telescope of his own made by Wm Herschel. He was supported in his proposal by the Chancellor of the University, the 4th Duke of Gordon, who had a lifetime's interest in science.

Curious watercolour 'primitive' sketch of King's College, said to be 1785.

AU Lemur image ABDUA 30407

By 1825, residence in College was no longer compulsory. As part of the re-furbishment, the six storeys of rooms were converted into three stories of class-rooms. (You can see the old 6-storey fenestration on the north wall. It is visible in old pictures only in the north and west walls). A new outside stair-well was added on the west side, the one you now climb to get to the top floor, and the building became what we see today. The two conical domes we now use sit atop this stair-well and were added then. The Professor of Natural Philosophy had his rooms on the top floor and is recorded as tending his 'philosophical instruments' on the roof of the tower in the 'astronomical room, reached by a winding stair', probably the stair we still use today. The King's College Minutes for December 1827 record the generous gift by Dr George Morrison of a valuable transit instrument presented to the observatory. Over the entrance to the staircase were placed the arms of Bishop Elphinstone, the founder of King's College who secured the authorising 'papal bull' in 1495. Nearby, on the outside wall at the foot of the tower is an inscribed bronze plaque to Alexander Forsyth, inventor of the percussion lock that revolutionised firearms at the beginning of the nineteenth century. Perhaps this military reference was intended as a reminder of Cromwell's days. Forsyth was a Minister of the Church in Belhelvie, a parish a little to the North.

West view of the tower before the addition of the staircase and existing domes, 1822

Cromwell Tower before the present adjacent building was erected. At this stage Mathematics was taught on the first floor and Natural Philosophy (i.e. Physics) class rooms were on the top floor

Cromwell Tower before 1860 from the rear.

Professor David Thomson

David Gill

As far as has been documented, the beginning of the modern use for the observatory was in the 1860s. The Professor of Natural Philosophy at the time was David Thomson. In 1860 the two separate Universities at Aberdeen were united into one 'University of Aberdeen'. David Thomson was encouraged to equip the domes as a modern observatory by a former student of James Clerk Maxwell, who had been Professor at the other college, Marischal College, in the 1850s. The student was David Gill, son of a local watchmaker and clock retailer who had a shop in Union Street, Aberdeen's principal thoroughfare. Gill was interested in 'time' and even more interested in Astronomy. Although he inherited the clock business from his father, he kept it only a few years before risking his career as an astronomer. It turned out to be a risk worth taking because he became after sometime Her Majesty's Astronomer at the Cape of Good Hope, Sir David Gill, KCB, a worthy successor to his predecessors in South Africa, and one of the most respected 19th century observational astronomers in the world. Gill's life is another story.

Gill's mean time clock

Astronomically, Gill's career started in the Cromwell Tower
2. In one swoop, Gill combined his interests in astronomy and time to bring astronomical time to Aberdeen. He persuaded Professor Thomson to set up the transit telescope that had been acquired years ago for the purpose but taken out of commission. This was mounted under the North dome. Gill installed a mean-time clock with electrical contacts, taking wires to the King's College turret clock, which was modified to act as a slave by the provision of a Bain's electrical pendulum. We still have the Gill's clock, with its clear silvered dial engraved 'D. Gill & Son, Watchmaker to the Queen, ABERDEEN'. The clock was readily adjusted to keep in synchronism with the heavens, any necessary corrections being made following observations through the transit telescope. Gill went further and took wires well over a km to the Marischal College tower clock and the best part of another km to the clock outside his own shop in Union Street. You can just see the residue of this wiring system in an insulator or two left on the other side of King's quadrangle. Even Edinburgh, with its one O'clock gun, couldn't match the technological innovation of bringing precision time to every citizen in the High Street, all day through. It was Edinburgh's one O'clock gun, set up by Piazzi Smyth, that had inspired Gill to suggest the whole enterprise to Thomson.

After the provision of time had been established as a routine, Gill and Professor Thomson looked to install an equatorial telescope under the south dome. After eyeing up a T. Cooke and Sons telescope that had become available upon the death of Cooke's intended customer, they had to settle for a second hand 3.75 inch diameter, 4 foot focal length refractor signed A. Ross, because the Cooke instrument was too big for the dome. Andrew Ross had died by then but his telescope had quite an early example of a clockwork tracking mechanism. The objective was reported by Gill to be good but the tracking mechanism imprecise. The telescope was used to follow the transit of Mercury across the face of the Sun on 4th November 1865 and observe the solar eclipse of 6th March 1867. You can still see this telescope in a display case in the foyer of the Fraser Noble Building, nearby. The telescope may have been good enough for teaching but Gill went on to pursue his own astronomical interests by purchasing a 12 inch silvered mirror and building his own reflecting telescope that he installed in his father's garden in the city. Street lights were not a problem in the late 1860s.

Thomson, meanwhile, turned the observatory's interest to meteorology and at the end of the 1860s managed to set up at the Cromwell tower the most northerly station in the first British national network of meteorological observing stations3. 7 stations covered Britain. (Apart form Aberdeen and Kew, these were Glasgow, Stonyhurst, Falmouth, Armagh and Valencia). They telegraphed readings of temperature, pressure and wind back to the 'Meteorological Committee' at Kew. The anemometer you see whirling beside the observatory, and blocking some of the view, is the original Robinson cup anemometer put in place in 1868 (or 1870 according to one source). There are very few such 4-cup instruments still in action on their original site. The Cromwell tower observatory was mainly used as a meteorological observatory from then on.

The Cromwell Tower anemometer against the skyline

The names of two observers span the next 75 years. William Boswell, who held the appointment from 1868 to 1902 and George Aubourne Clarke, who was observer from 1903 to 1943. George Aubourne Clarke became famous for his cloud photographs taken from the Cromwell tower, so much so that they were used for decades by the Met Office, the Admiralty and other organisations throughout the world to illustrate the typical forms of clouds. Beneath the pictures of clouds you will see recognisable views across the dunes out to sea, to the east, and in other pictures the twin towers of St Machar's cathedral a little to the West of North. You will find a good number in GAC's book4, where he finishes the preface with his address as "The University Observatory, King's College, Aberdeen". George Aubourne Clarke's pictures were really only succeeded with the widespread introduction of colour photography.

George Aubourne Clarke



It took a combination of research and inspiration to deduce that this photograph (above left) was shot at about 7.45 am on Wednesday 29th June 1927. Beside the long-focus bellows camera is George Aubourne Clarke FRPS, Aberdeen's brilliant cloud photographer; at the table adjusting the silver-disk pyreheliometer, Owen F.T.Roberts, lecturer in astronomy, assisted at the clock by a youthful Charles Strachan, whose association with our university spanned almost seventy years . They have just observed from the roof of the Cromwell Tower the first total eclipse of the sun (above right - what was seen) that happened in the 20th century where totality could be seen from some parts of mainland Britain. In Aberdeen it was only a partial eclipse.

In 1921 the observatory was taken over by the Air Ministry. Dyce airport didn't exist then. The Air Ministry staffed it under GAC's supervision until he retired, and then finally closed down their operation in 1947. When I first saw into the observatory in the 1960s, it looked as if it hadn't been entered since then. Much later, the historical equipment still there, most dating back to the nineteenth century, was saved for our museum. Michael Gadsden used the observatory from the 1970s to the 1990s as a station for observing noctilucent clouds. He erected an 'all-sky' camera on the roof outside and equipment for measuring the altitude and azimuth of cloud features. For two winter seasons, 1995/96 and 96/97, the National Physical Laboratory used the observatory to measure concentrations of molecular species in the atmosphere by the use of a high resolution Fourier Transform spectrometer looking at sunlight travelling at low angles through the atmosphere. In 2000, we began a refurbishment programme, aimed to bring the observatory back into astronomical use and in 2002 we began what we hope will be a long-standing co-operative venture with the Aberdeen and District Astronomical Society. A 10 inch diameter Meade Schmidt-Cassegrain reflector has been installed in place of the Ross telescope but a leaking roof that is expensive to repair and floodlighting that has been installed to illuminate the King's College facade has considerably reduced the usefulness of the observatory in recent times.

The National Physical Laboratory fourier transform spectrometer in the tower to monitor greenhouse gas IR absorption.

1 C. B. R. Butchart The Cromwell Tower, 1658 - 1958, The Aberdeen University Review, vol. 38, pp 143 - 147, 1960.
2 David Gill History and Description of the Royal Observatory Cape of Good Hope, pp xxx - xxxii, Neill & Co., Edinburgh 1913.
3 A. E. M. Geddes The Development of the Study and Practice of Meteorology at Aberdeen, Weather, vol 10 385 - 389, 1955.
4 George Aubourne Clarke Clouds: a descriptive illustrated guide-book to the observation and classification of clouds, Constable & Co., London 1919.

All pictures courtesy of University of Aberdeen Natural Philosophy Historical Collection or John S. Reid, except where noted.

John S. Reid, Department of Physics, University of Aberdeen

Updated September 2013

This page has been translated into Finnish by Fijavan Brenk

Aberdeen University has a prestigious collection of historical objects, each of which has played a part in the education of students of Natural Philosophy over the past 250 years. The Natural Philosophy Collection of Historical Scientific Instruments is curated by Dr John S. Reid.


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