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Received My 22, 1772. 
XV. Remarks on the Hadley’s uadrant* 
tending principally to remove the Difficulties 
which have hitherto attended the Ufe of the 
Back- obfervation^ and to obviate the 
Errors that might arife frotn a JVant 
of Parallelifm in the two Surfaces of 
the Index-Glafs. By Nevil Mafkelyne, 
F. R. S. Aftronomer RoyaL 
Read May 28, Y' | "1 H E back obfervation with Hadley’s 
quadrant being founded on the 
fame principles, and in theory, equally perfect with 
the fore- obfervation, and being at the fame time 
neceflary to extend the ufe of the inftrument up to 
180 degrees (it being impracticable tomeafure 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 pradtifed 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 obfer- 
