VOL. LXV.] PHILOSOPHICAL TRANSACTIONS. 713 



shown in its favour. Besides, it might be expected that some unsteadiness or 

 warping of the wooden stand, on which it was supported, might affect the 

 accuracy of the observations ; or there might be variable and discordant refrac- 

 tions, even near the zenith, on the side of so steep a hill, more than are found 

 in lower situations. Add to this, that when I began my observations on the 

 south side of the hill, having a prospect of bad weather before me, and not 

 knowing how few observations I might be able to get on either side of the hill, 

 I thought it prudent to endeavour to observe most of the stars in the British 

 catalogue, which came within the reach of the instrument, that I might be sure 

 of being provided with observations of some at least of the same stars, which 

 I might afterwards observe when I should be removed to the north side of the 

 hill ; where, after an interval of perhaps some months, many stars, that before 

 passed the meridian in the night, would pass it in the day, and consequently be 

 either invisible through the telescope of the sector, or more precarious of being 

 seen. 



Though a meridian line had been found by the transit instrument at the south 

 observatory, by which the relative situation of the 2 stations of the observatory, 

 as well as of the other points of the hill, with respect to the meridian, might 

 be determined ; yet I judged it would be more satisfactory to confirm this by 

 another meridian line drawn at the northern observatory. This I found, as I had 

 done the former, by setting the transit instrument to agree with the pole star 

 at the computed time of its passing the meridian, and confirmed it by comparing 

 the difference of the transits of the pole star and of a Pegasi, « Andromedse, 

 and y Pegasi, with their difference of right ascension, in the same manner by 

 which I had put the sector in the plane of the meridian, and found it to agree 

 with the former meridian line within 1 minutes. 



It remains to give an account of the manner in which the two bases were 

 measured ; one in a level spot at the foot of the hill, to the southward ; and 

 the other at the distance of about 1\ miles from the hill to the north west, in 

 the plain of Rannoch. I caused 2 measuring poles to be made of straight 

 grained well seasoned fir, in the form of square tubes, 3 inches square and 20 

 feet long, and strengthened with square pieces within side at several distances. 

 These were carefully compared with the brass standard made by Mr. Bird, the 

 same which was used in the measure of the degree at Pennsylvania, immediately 

 before they were applied to the measure of the bases, and the height of the 

 thermometer noted at the time, in order to make allowance for the expansion or 

 contraction of the brass standard by heat or cold. Four wooden stands were 

 provided for supporting the poles ; each having a triangular base with 3 iron 

 spikes beneath, at each of the angles. An upright pole, 6 feet high, rose 

 from the middle of one side of the triangle, and 2 short braces were joined to it 



VOL. XIII. ..,,,, 4 y . . • 



