38 OBSERVATIONS of the Places, &c. 



92'. The planet was always compared with at leaft two ftars, 

 which pafTed through the field without altering the pofition of 

 the inftrument. The interval between the tranfits of the fixed 

 ftars, compared with their difference in right afcenfion in the 

 tables, fhows the error of the pofition of the horary wire ; and 

 the planet's difference in declination fhows what portion of this 

 error is to be applied to the time of its tranfit. When the pofi~ 

 tion of the horary wire was very oblique to the horizon, and 

 the altitudes fmall, a corredlion was made for the difference in 

 refradlion. 



Both ends of the polar axis were firmly fupported in a 

 ftone wall. The telefcope turned round on a pin within two 

 inches of the upper pivot of the axis, and clofe by the objedl 

 glafs. The other end of the telefcope was fupported (at the 

 place of the wires) by a lliff rod, which turned round a pin 

 within two inches of the lower end of the polar axis ; fo that 

 the telefcope, axis and this rod, formed a triangle. Another 

 ftiff rod was faftened to the telefcope at the place of the wires, 

 with a double joint, and its other end palTed through a focket, 

 firmly fixed on the fide of the window, where it was held faft 

 by a fcrew-pin. The rod was in a plane, nearly parallel to the 

 equator. It is eafy to fee that, by this conftrudion, each part 

 of the inftrument was expofed to a longitudinal ftrain alone, 

 and all effedts of the tremor of its parts were avoided. It was 

 fo completely free from any inconvenience of this kind, that, 

 even in very boifterous winds, the image of the ftar was per- 

 fe(flly fteady, and free from every kind of quivering. I never 

 found any two comparifons of the planet with the fame pair of 

 ftars differ above half a fecond in time. As the inftrument was 

 fo exa<3:, and did not (exclufive of the telefcope) coft above 

 three pounds, I thought that this fliort account of it would be 

 acceptable to fuch as are not provided with thofe expenfive in- 

 ftruments which are thought effentlally neceflary for making 

 good and ufeful obfervations. 



VIII. 



