226 FHILOSOPHICAL TKANSACTIONS. [anNO 1777. 



XXXI f^. Experiments and Observations made in Britain, in order to obtain a 

 Rule for Measuring Heights with the Barometer. By Colonel JViUiam Roy* 

 F.R.S. p. 653. 



Ever since the discovery made by Torricelli, the barometer has been applied, 

 by different persons, in different countries, to the measurement of vertical 

 heights, with more or less success, according to the state of the instruments 

 used, and the particular modes of calculation adopted, by the observers. But of 

 all those who have hitherto employed themselves in this way, none has bestowed 

 so much time and pains, or succeeded so well, as Mr. De Luc, of Geneva, 

 p. R. s. In two quarto volumes, published some years since, that gentleman has 

 given us the history of the barometer and thermometer, with a very curious and 

 elaborate detail of many years experiments, made by him, chiefly on the moun- 

 tain Saleve. Now the rule, deduced from the observations on Saleve, consists 

 of 3 parts. 1st. The equation for the expansion of the quicksilver in the tube, 

 from the effect of heat, by whicli the heights of the columns, in the inferior 

 and superior barometers, are constantly reduced to what they would have been 

 in the fixed temperature of 54^° of Fahrenheit, independent of the pressure 

 they respectively sustained. 2d. When the mean temperature of the column of 

 air to be measured, is 69°. 32, as indicated by thermometers exposed to the sun's 

 rays at its extremities ; tlien the difference of the common logarithms, of the 

 equated heights of quicksilver in the two barometers, gives the altitude inter- 



* Col. Will. Roy died July J, M9^), at his liouse in Argyle-street, after only 2 hours illness : his 

 age not known. At the time of his death he was deputy quarter-master general, colonel of the 3C>tli 

 regiment of foot, surveyor-general of the coasts, major-general in the army, and a very respectable 

 member of the Royal and Antiquarian Societies. General Roy was a native of Scotland, and had 

 formerly been an officer in die royal artillery, but changed on promotion into the line. Besides tlie 

 present elaborate paper on the measurements of altitudes by the barometer, which is his first in the 

 Philos. Trans, he has 3 other valuable communications in tlie same Trans, vols. 75, 78, and SO j the 

 former on the measurement of a base on Hounslow-heath, for w hich he was honoured with the 

 K. s.'s gold medal ; and tlie latter on the relative situations of the obsenatories of Greenwich and 

 Paris. General Roy had been employed, at difterent times of his long life, on several important 

 public works ; in the winter of 1746 he made an actual survey of Scotland, called ilie duke of Cum- 

 berland's map, which was on a very large scale, and deposited in the Ordnance office ; tliis he after- 

 wards reduced to a smaller size, and had engraven, under the title of the King's Map. In 1784 he 

 was employed on tlie measurement of the base on Hounslow-heath, abovementioned, as preparatory 

 to the Trigonometrical Survey, afterwards accomplished, for determining the relative situations of the 

 two national observatories of France and England, noticed above. By command of his majesty, he 

 had undertaken and just completed a most ciu-ious, accurate, and elaborate set of trigonometrical ex- 

 periments and observations, to determine tlie exact latitude and longitude of those two observatories ; 

 s«i account of which, illustrated by tables computed from actual measurements, he had drawn up, 

 and presented to the r. s. and was superintending the printing of it at tlie time of his death, which 

 happened as before mentioned. This account of the operations was printed in the Phil. Trans, vol. 

 80, for the year 1790. 



