708 I'HILOSOPHICAL TRANSACTIONS. [aNNO 1775. 



it may be safely concluded, that the instrument is truly placed in the meridian. 

 If not, by comparing the alteration that would be produced in the difference of 

 the transits, by supposing the instrument out of the meridian, by any small 

 quantity, as 1 degree or ]0 minutes with the observed error, the deviation of 

 the instrument from the meridian may be inferred. In this manner I found that 

 the instrument had been set very exactly in the meridian, by means of the meri- 

 dian line; the difference by the two methods coming out only 2-1- minutes of 

 azimuth. As to the continuance of the instrument in the plane of the meridian, 

 I had a constant proof of it by the same means, and also a further security, 

 which I did not fail to attend to, by noting the degree and minute which an index 

 depending on the vertical axis of the instrument pointed out on a fixed azimuth 

 circle. Being apprehensive of error in an instrument supported on a wooden 

 frame, I frequently examined the parallelism of the fore arch to the back arch, 

 by measuring their perpendicular distances at the two ends with a brass scale, 

 whose vernier showed the SOOth part of an inch, and found it liable to variations 

 of a minute or two, owing probably to the force used in setting the sector to 

 different zenith distances, and the weakness of some screws at the top of the 

 frame; which small error I corrected, till I found it liable to continual returns: 

 and I satisfied myself, that the plane of the sector never deviated above 3 mi- 

 nutes from the meridian, in any of the observations taken on the south side of 

 the hill, which it is evident could not in the least affect the observed zenith disT 

 tances of stars. I hardly ever observed, without examining the bisection of the 

 point at the centre of the instrument, by the plumb-line; which was absolutely 

 necessary, on account of the gradual changes of the woollen frame. My view 

 in mentioning these minute circumstances, is to caution future observers, as well 

 as to confirm my own observations. But whoever makes use of an instrument of 

 this kind, supported on a wooden frame, will find the greatest attention necessary 

 to attain the same degree of accuracy in his observations, as if his instrument 

 was fixed to an immoveable wall. In the mean time, by observations taken with 

 the quadrant and transit instrument, I got a meridian line, and planted a pole to 

 preserve it on the top of the hill, to the south of the instrument, and another 

 at the foot of the same hill ; from which, by measuring off an equal distance to 

 the east (as the south-west corner of the observatory lay to the east of the transit 

 instrument) and setting up another pole, another meridian line was gotten, pass- 

 ing through the south-west corner of the southern station of the observatory. 

 The reason for making the meridian line pass through the south-west corner of 

 the observatory, rather than through the middle of it, was, that this part of it 

 had been taken when the observatory had been used as an object in taking angles 

 by the theodolite, in the survey of the hill. 



While I was engaged in these astronomical observations, Mr. Burrow, at- 



