[ 559 ] 
M Y Hadley’s quadrant, which was made by Mr. 
Bird, the radius of which is 20 inches, appears, 
from all the trials I have made with it, to be very 
exaCl; this, indeed, the obfervations of the Moon 
alone fufficiently prove, fmce I have often taken the 
didance of the Moon from two dars, on different 
tides, on the fame night, and found the fame longi~ 
tude to refult from thefe feparate obfervations, with 
as little difference as from two obfervations of the 
fame dar ; whereas, if there was the lead; error in 
the divifion of the quadrant, or the lead refraction in 
the glaffes of it, the error arifing from hence affect- 
ing the computation of the longitude from the two 
feparate obfervations contrary ways, mud: be fenfible 
in the refult. I was fecured from any errors in the 
condruCiion of the quadrant, by the known fkill of 
the artift ; and the fpeculums, as well as dark gladfes, 
were ground by Mr. Dollond, by a particular method 
of his own, by which he is certain of making the two 
furfaces of a glafs truly parallel to each other. 
The arch and the index were both of brafs, and 
the frame of well feafoned mahogany; the Verniers 
fcale on the index carried the fubdivifions to fingle 
minutes ; and the eye might fubdivide ftill nearer to 
the fradion of a minute. When the index was 
brought near the proper didance for cbfervation, in 
order to give it a more deady motion than the hand 
alone 'could, a plate was fcrewed down to the arch, 
containing the head of a fcrew, by turning of which, 
the index was carried backwards or forwards at plea- 
fure. This contrivance was extremely neceflary for 
taking the didance of a dar from the Moon’s limb in 
Vol. L 1 I. 4C an 
