i 560 ] 
an exadt manner ; for the quadrant being often to be 
held in a very inclined plane, it was impoffible, on 
account of the motion of the fhip, to make the ob- 
fcrvation at once ; therefore, by means of the fcrew, 
the index was moved by gentle degrees, and each 
time after altering the fcrew, the quadrant was turned 
by the hand round the vifual ray, going to the ftar as 
an axis, fo as to make the ftar feem to pafs by the 
Moon’s limb, and the index was gradually moved, 
till at laft the ftar, in paffing by, exactly fwept the 
Moon’s limb. Befides the fcrew moving the index, 
there was another addition no lefs neceftary, which 
was a dark glafs, much brighter than any of thofe 
commonly ufed, to take off the exceflive brightnefs 
of the Moon, and that glare upon the horizon-glafs, 
which always attends it. Without the ufe of this 
glafs, it was not eafy to obferve accurately the diftance 
of a ftar from the Moon’s limb, except when the 
Moon’s light was weakened by a thin cloud. 
In order to render the contadt of a ftar with the 
Moon’s limb more difcernible, I always ufed a fmall 
telefcope, magnifying about four times, except when 
the motion of the fhip was very troublefome, though 
I reckoned the obfervations were not quite fo exadt as 
thole made with the telefcope. The telefcope, by 
means of two fwivels adapted to it, had a motion 
parallel to itfelf, by which it was carried nearer to or 
further from the plane of the quadrant ; by means of 
which, the Moon, which was feen always by re- 
flexion, was rendered more or lefs bright, according 
as more or lefs of her light was received by the tele- 
fcope, from the quickfilvered part of the objedt fpe- 
culum 5 or by railing the telelcope ftill higher from 
1 > die 
