IN FORT WILLIAM, &C. 6l 



obferved. I found it fo experimentally, and corrected 

 my quadrants accordingly, by turning the horizon glafs 

 round its own axis; then having adjufted as ufual, the 

 error fideways was corrected by moving both glaffes, 

 by means of their adj lifting fcrews, and dividing the 

 error between them. If, when the horizon glafs was 

 reflored to its proper pofition, there itill was a lateral 

 error, the operation was repeated. I do not find any 

 mention of this in any of the inftructions for ufing 

 Had ley's inftruments that I have feen. 



The horizon was artificial, invented for the occafion, 

 and confided of a wooden trough about ~ an inch deep, 

 (or rather more,) filled nearly with quickfilver, which 

 ferved to float a plate of thick glafs, the under furface 

 of which had been unpolifhed, and blacked, that only 

 one image might appear. This needs not any adjuft- 

 ment : the only requifite is, that the glafs be equally 

 thick all over, and fmooth : that which was ufed was a 

 part of a very large looking-glafs, that had been broken 

 by accident. 



The watch was a time-keeper, by Brookbank, which 

 goes whilft it is wound up, and is tolerably good, con- 

 fidered as a fale watch lent to India. 



The telefcope had a double object glafs, with a brafs 

 ftand, and was made by Gregory: it magnifies 80 

 times; but, like all of this conftruction, that I have 

 feen, it had a dark fpeck in the middle, and was not 

 equally good in the whole field. 



In the way back, wehad a land quadrant, of 15 inches 

 radius, made by B. Martin, and fent out by the India 

 Company. It was ufed by Mr. Hurst in the tranfit of 

 Venus. This could not be inverted. But, to deftroy 

 the effects of collimation, and error of level, the lati- 

 tudes are all determined by ltars taken north and fouth 

 of each place, as the obfervations will (hew. 



T. D. PEARSE. 



