[99] 



Received May 22, 1772* 



XV. Remarks on the Hadley's Quadrant* 

 tending principally to remove the Difficulties 

 which have hitherto attended the life of the 

 Back-obfervation^ and to obviate the 

 Errors that might arife from a Want 

 of Parallelifm in the two Surfaces of 

 the Index- Glafs. By Nevil Mafkelyne, 

 F. R* 8. Aftronomer Royal. 



Read May 28, fr j "H jj £ back obfervation with Hadley's 

 JL quadrant being founded on the 

 fame principles, and in theory, equally perfect with 

 the fore-obfervation, and being at the fame time 

 necefTary to extend the ufe of the inftrument up to 

 1 80 degrees (it being impracticable to meafure angles 

 with any convenience beyond 120 degrees with the 

 fore-obfervation) it may feem furprizing that it hath 

 not been brought equally into general ufe, more e- 

 fpecially fince the method of finding the longitude 

 by obfervations of the Moon, has been practifed at 

 fea for fome years paft ; fince this method would re- 

 ceive confiderable advantage from the ufe of the 

 back-obfervation in taking diftances of the Sun and 

 Moon between the firft and laft quarter, could fuch 



O 2 ofcfer- 



