VOL. LXV.J PHILOSOPHICAL TRANSACTIONS. 703 



traction of some hill in this kingdom by astronomical observations, to the R. s.; 

 who, ever inclined to promote useful observations which may enlarge our views 

 of nature, honoured it with their approbation. A committee was in consequence 

 appointed, of which number I was one, to consider of a proper hill on which 

 to try the experiment, and to prepare every thing necessary for carrying the de- 

 sign into execution. The Society was already provided with a 10-feet zenith 

 sector made by Mr. Sisson, furnished with an achromatic object glass, the prin- 

 cipal instrument requisite for this experiment, the same which I took with me to 

 St. Helena in the year 176] ; which wanted nothing to make it an excellent in- 

 strument but to have the plumb-line made adjustable, so as to pass before and 

 bisect a fine point at the centre of the instrument. This was ordered to be done, 

 and a new wooden stand provided for it, capable of procuring a motion of the 

 sector about a vertical axis, by means of which it could be more easily brought 

 into the plane of the meridian, or turned half round for repeating the observa- 

 tions with the plane of the instrument placed the contrary way, in order to find 

 the error of the line of collimation. A large parallelopiped tent, 15-i. feet square 

 and 1 7 feet high, was also provided for sheltering both the instrument and the 

 observer who should use it, composed of joists of wood well framed together, 

 and covered with painted canvas. The Society was also possessed of most of the 

 other instruments requisite for this experiment; as, an astronomical quadrant 

 and transit instrument made by Mr. Bird, and an astronomical clock by Shelton, 

 which had all been provided on occasion of the observations on the transit of 

 Venus in 1761 or 1769. A theodolite of the best sort was wanting, a necessary 

 nstrument for obtaining the figure and dimensions of the hill. One of Mr. 

 Ramsden's construction of Q inches diameter, was thought the fittest for the 

 purpose, on account of the excellence of the plan on which it was made, and 

 the number of its adjustments, being capable of measuring angles for the .most 

 part to the exactness of a single minute. The other instruments prepared for 

 this business were, 2 barometers of M. de Luc's construction, made by Mr. 

 Nairne; a common Gunter's chain; a roll of painted tape 3 poles long, having 

 feet and inches marked on it; 2 fir poles of 20 feet each, and 4 wooden stands, 

 for supporting them when used in measuring the bases, and a brass standard of 

 5 feet, for adjusting them. The poles and stands were provided on the spot. 



Though accounts had been received from various persons of several hills sup 

 posed proper for the intended purpose, some better and some worse authenticated; 

 yet, in order to be sure of finding the best hill for the experiment, it was deter- 

 mined to send a person furnished with proper instruments, to make such obser- 

 vations on various hills in England and Scotland, as might enable us to choose 

 the fittest for the purpose. Accordingly Mr. Charles Mason, who had been 

 employed on several astronomical occasions by the r. s., was appointed to make 



