204 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777. 



quite 3 miles in a horizontal line from the top of the rock, whose height was to 

 be determined. At the end a, of the base ab, he intended to place one of the 

 barometers ; and the other at the top of the rock, called the Pitton, at c the 

 highest point of Mount Saleve ; and with proper instruments to measure the 

 triangle abc. The angles were taken both on the horary circle, which was 

 brought parallel to the horizon, and also on the azimuth circle of the equatorial 

 instrument; this made it, as it were, 2 different instruments independant of 

 each other. The angles were also doubled, tripled, and quadrupled, on each 

 arch ; by this means the error of the centre or axis of the instrument vanished ; 

 the possible error in the divisions, in the reading off, and in the coincidence of 

 the wires in the telescope (which magnified 40 times) with the signals placed at 

 each angle of the triangle, was lessened in proportion to the number of times 

 the observation was repeated ; and finally the mean of all was taken. The same 

 was done with each angle at a, b, and c, horizontal as well as vertical, viz. the 

 elevation of c above a and b was taken ; and also the depression of a and b 

 below c. The advantage of this method was, that the error of tlie line of col- 

 limation, the effect of refraction, and of the curvature of the earth's surface, all 

 became equal and contrary ; by these means the little errors were diminished, 

 and great errors absolutely avoided. 



From this measured base, and the angles taken at its extremities, by trigo- 

 nometrical calculation Sir Geo. S. determined the sides of the triangle abc, 

 with the relative altitudes of its angular points, and consequently the height of 

 the mountain at its highest point c ; which were as below, viz. the side ab 2760.8 

 feet, AC 15286.4, BC 14041.7; and the height of c, the highest point, above 

 A the lowest, 2831.3 feet, which is probably within 3 or 4 feet of the truth, or 

 about 1 foot in 1000. 



Having thus the perpendicular height very accurately ascertained by geometry, 

 it remained to take the altitude of the barometer at each station a and c, and if 

 possible with equal precision. These observations it would be too tedious to set 

 down at length. The following particulars however may be noticed. Every ob- 

 servation of the barometer was triple ; that is, the height was read off 3 different 

 times, and the mean taken ; but fro-n once reading only Sir G. could be sure of 

 the height to -roVo of an inch, exclusive of the error of the divisions, which in 

 some places might amount to that quantity ; this the nonius would itself discover 

 and even correct by estimation. At every series of observations the float at the 

 bottom was readjusted, so that he could constantly be sure of an alteration of the 

 weight of the atmosphere expressed by 0.002 inch of quicksilver, if not of 

 half that quantity. Finally, the difference of the two barometers was constantly 

 taken, after being left 4 of an hour or more in the same place, to acquire the 

 true temperature of the air, and this before and after every expedition. Hence 



