i 96 ] 



has oftentimes been defeated by the index glafs being 

 bent by the brafs frame that contains it: to prevent 

 this, I have contrived the frame, fo that the glafs 

 lies on three points, and the part that preftes againfl: 

 the front of the glafs has alfo three points exactly 

 oppbiite to the former. Thefe points ate made to 

 confine the glafs by three fcrews at the back, that 

 act exactly oppoftte to the points between which the 

 glafs is placed. This little contrivance may be of fome 

 ufe y but the principal improvements are in the me- 

 thods of adj lifting the glaffes, particularly for the 

 back obfervation. 



The method hitherto pra&ifed for adjufting that 

 part of the inftrument, by means of the oppofite 

 horizons at fea, has been attended with fo many 

 difficulties that it has fcarce ever been ufed ; for fo 

 little dependance 'could be made on the obfervations 

 taken this way, that the beft Hadiey's fextants made 

 tor the purpofes of obferving the diftances of the 

 Moon from the Sun or fixed ftars, have been always 

 made without the horizon glafs for the back obfer- 

 vation ; for want of which, many valuable obferva- 

 tions of the Sun and Moon have been loft, when 

 their diftance has exceeded 1 20 degrees. 



To make the adjuftment of the back obfervation 

 eafy and exact, I have applied an index to the back 

 horizon glafs, by which it may be moved into a 

 parallel poiition to the index glafs, in order to give 

 it the two adjuftments, in the fame manner as the 

 fore horizon glafs is adjufted. Then, by moving 

 the index to which the back horizon glafs is fixed, 

 exactly 90 degrees (which is known by the diviiions 

 made for that purpofe) the glafs will be thereby fet 

 6 at 



