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Accuracy and Experiment - Examples from Salisbury Cathedral

When the spire of Salisbury Cathedral was completed in about 1310 it was the tallest stone structure in Europe and it remained so for a hundred and fifty years. After the demise of Lincoln and Old St. Pauls it was the tallest building in England.

The Cathedral was built between 1220 and 1310 and such was the accuracy of its builders that from a centre line laid east to west no measurement to piers or outer walls varies by more than 5/8ths of an inch.

When levelled across the main transepts, a distance of some 210 feet, it proved to be just half an inch out of level - this after 750 years of settlement. Such accuracy is not too difficult to achieve, yet many present day buildings might not stand comparison with such standards. The tools and equipment used by those 13th century builders were much the same as today's, except for the absence of mechanical power. Masons' tools in particular have changed little over the centuries and a 13th century craftsman would find no difficulty in using modern hand tools. Even chisel sizes, checked from the tooling marks on the finished stone, match today's standards. A small army of masons, carpenters, plumbers, glaziers, plasterers and painters, using the best technology of the period, built the cathedral, and a similar, although much smaller army is constantly engaged in repairing and conserving it today.

tower and spire

The tower and spire of Salisbury Cathedral. The weather door is at a level just above the uppermost of
the three levels of decorative stonework.

Technology has changed: one person with a machine can move blocks of stone weighing several tons; diamond edged power saws make easy work of cutting away much of the waste that once had to be chipped away by hand. Yet there is little difference to be seen between the finished stonework of the 13th and the 20th centuries.

The great cathedrals of the Middle Ages were at the forefront of technological progress, changes in architectural style for instance extending the scope of the craftsman and the daring of the designer. Gradually the focus of attention changed from ecclesiastical to secular buildings, coinciding with a greater interest in scientific investigation.

One such scientific experiment, to check the effect of height on air pressure, and thus to demonstrate the barometer as an instrument to measure height, was carried out at Salisbury in 1684. It was first recorded in the proceedings of the Royal Society of London for that year after John Aubrey had related the facts to the Society1. More detail is given in a book published in 17742, based on earlier work by Francis Price, where, in the section dealing with the spire, it is stated that Colonel John Wyndham in 1684 had calculated the height with the aid of a barometer. At the weather door3 he had noted that the mercury had dropped by 42/100ths of an inch from the level it had been at the floor. From this he deduced that the weather door was 4,280 inches up and thus, after adding the known distance of 40 feet from the weather door to the top of the spire, arrived at a total of 404 feet.

On the 17th of January 1685 William Musgrove4, then Secretary of the Royal Society, wrote to a Mr Aston. Amongst the items in the letter is a mention of 'some discourse at the Philosophical Society at Oxford concerning the barometer being used as a level, to discover the difference between the several heights of places distant from one another.'5

letter warner to boyle

The part of the letter sent by John Warner to Robert Boyle in 1685 showing the drawing in the margin. The original letter is in the library of Worcester College, Oxford.

In a letter dated November 25th, 1686, from John Wallis6 to Edmund Halley7 mention is made of the Salisbury experiment:

'We had a particular account of an observation made at Salisbury, in November 1684, by Colonel Wyndham and Mr Warner, of the altitude of quicksilver in a baroscope, at several heights between ground and the top of the spire; and at what proportions it decreased'8.

W.E. Knowles Middleton, in his History of the Barometer9, refers to the work by Robert Boyle10 in connection with air pumps and the properties of gaseous materials. Boyle had repeated experiments carried out by Torricelli11, one of them in the presence of the then Bishop, Seth Ward and his consultant architect, Christopher Wren. Wren had suggested barometric experiments to test the effect of tidal movement on atmospheric pressure but Boyle declined that suggestion.

It is clear that both the Bishop, at the time Wyndham carried out his experiment, and his consultant architect, had more than a passing interest in barometric research since both were members of the Royal Society and both had been present when Wyndham's experiment was discussed. Unfortunately it is not recorded what type of barometer was used, the simplest type being a glass tube, sealed at one end, filled with mercury, the open end temporarily plugged and the tube inverted. The open end is then lowered below the surface of a container of mercury before the temporary plug is removed. Air pressure on the surface of the mercury in the container will maintain a column of mercury in the tube some ten inches high. The space in the tube above the mercury is known as the Torricellian Vacuum. Any change in the air pressure is indicated by a change in the height of the mercury in the tube. Air pressure will vary under the influence of the weather, or according to the height above sea level. Wyndham's experiment at Salisbury was one of the first concerning the measurement of height and it was not until a century later that Jean Deluc (1727-1817)12 became the first to record accurately the height of mountains by use of a barometer.

There have been other experiments in Salisbury Cathedral, not to determine the height of the spire but to find out how much it deviates from the plumb. Christopher Wren carried out his well-known inspection of the cathedral in August 1668, including some taking of measurements. His method was simply to hang a plumb line in the most accessible part of the base of the spire and in the tower, taking accurate measurements to many points at various levels and then calculating the amount of declination. He judged it to be 27½ inches to the south and 17½ to the west. In 1681 Thomas Naish repeated the exercise and found the declination to the south to be 24 inches and to the west 16 inches, In 1736 William Naish and Francis Price plumbed the tower and spire and marked the pavement at the crossing to show the amount of declination towards the south west In 1737 James Mill of London took a measurement. After plumbing the tower and spire he set up plumb lines on framework outside two of the main doors and used a lighted candle as a moveable sighting mark at the crossing to establish the point of intersection.

The following report, by John Warner, Colonel Wyndham's assistant, is quoted from "The General History of the Air" by the Honourable Robert Boyle, Esq, Imprimatur June 29 1692. The exact functioning of the "inverted Baroscope, like that figure in the Margin" (Fig.1) is not clear, but it evidently had the effect of giving higher, rather than the usual lower, readings for the mercury level as the height increased.

Experiment made at the Spire of the Cathedral Church in Sarum, by Colonel John Wyndham, assisted by Mr Theo. Naish, Clerk of the Works and John Warner in November 1684.

Having gotten together all the surveighing chains the city afforded, and carefully examined their truth and having prepared a proper frame for the Baroscope, we went into the church, filled the tube, and with all the nicety we could refer purged it of the airy particles and then immersing it, as in the Torricellian experiment; the mercury was then suspended 30 inches, and 50 cents of an inch, measuring it from the surface of the Hagnum, Then drawing it up to the first floor above the vaulting, which is 1033 inches and ½ inch from the pavement, the mercury subsided 9 cents of an inch: from thence drawing it up to the middle floor which is 935 inches higher the mercury subsided 8 cents lower than before: and from thence drawing it up to the weather door; which is 2313 inches higher than the last, the mercury subsided 23 cents below its last station. So the whole height 'twas drawn up, is 4281 inches and a half. And the whole difference of the mercury's standing is 40 cents of an inch, And letting it down the same way, the mercury reascended to its first station.

At another time with an inverted Baroscope, like that figure in the margin, having made a mark where the liquor stood when 'twas below in the church and drawing it up to the first floor over the vaulting.

If your honour desires to have any other Experiments, made at the Spire, Mr Naish whom I have mentioned above, is a person well skilled in the practical mathematicks, and a great lover of learning, but more especially natural and experimental phylosophy, having all or most of your Honour's Phylosophical works. This person I know would most gladly embrace any opportunity of serving your honour whensoever you'l be pleased to let me impart anything to him in a letter.

This is humbly advertised by Sir
Your Honour's most obliged and most Obedient Servant
John Warner
4 December 17
Anno 1685

Experiments still go on. Today they are more concerned with methods of conservation rather than measurement of the tower and spire, although I have myself used a surveying aneroid to check the height of the spire and can only agree with Wyndham's finding: it is 404 feet.


1 "The History of the Royal Society of Landon, Vol.IV" by Thomas Birch, 1774, p. 348.

2 "A Description of that Admirable Structure the Cathedral Church of Salisbury". Price was Clerk of the Works to the Dean and Chapter.
3 The weather door is on the north face of the spire, 40 feet below the top, just above the uppermost decorative band. Ascent above that level has to be made on the outside of the Spire by climbing the iron rungs get in the stone.
4 William Musgrave, 1657-172l, physician and antiquary, secretary of the Royal Society 1685.
5 History of the Royal Society of London, Vol. IV p. 508.
6 John Wallis, 1616-1703, mathematician, Savilian professor of geometry at Oxford.
7 Edmund Halley 1656-1742, astronomer. He followed Wallis as professor of geometry at Oxford. Predicted the return of the comet which bears his name.
8 History of the Royal Society of London, vol. IV p. 508.
9 The John Hopkins Press, 1964.
10 Robert Boyle, 1627-1691, engineer, chemist and philosopher. Established Boyle's Law; introduced colour testing for acidity and alkalinity; introduced hermetically sealed thermometers, and much more.
11 Evangelista Torricelli, 1608-1647, one time secretary to Galileo.
12 Jean Deluc, 1727-1817, Swiss geologist, meteorologist and physicist.

This article by Roy Spring originally appeared in TATHS Journal 11, 1999.

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