[ 3S9 ] 
are placed at exaft right angles to each other (which 
they feldom are) and, if they are not, what the 
quantity of deviation is, in order to make an al- 
lowance for it in the reduction of the obfervations. 
This may be tried feveral ways. I examined the 
wires of my telefcope at Barbados, by a gieat many 
obfervations of the difference of right aicenfion of 
ftars, which differed confiderably from each other 
in declination, namely Ardurus, and the little fiar 
accompanying it, and the virgin s fpike, and a fmall 
flar preceding it j fir ft with the wires in the common 
pofition, and next when turned a quarter round, 
makincr the middle horary wire ferve as a directing 
wire • for, if the wires do not cut each other at light 
angle’s, the difference of right afcenfion of the 
ftars will come out too much one way, and as much 
too little the other way, and half the difference will 
be the corredion in this cafe, whence it may be in- 
ferred in all other cafes. Or, the angle of the de- 
viation of the wires from a right angle being hence 
found the correction of the difference of light 
afcenfion of the moon and ftar, is to the difference 
of their apparent declination ; as the fine of the angle 
of the deviation of the wires is, to the cofine of the 
moon or ftar’s declination. . 
I have determined the deviation of the wires of 
the telefcope, which I ufed at St. Helena, by com- 
paring them with a right angle, formed by two 
Elver Vires on a brafs plate, fixed up in a window at 
the diftance of 30 feet from the telefcope The 
extent of the compaffes, with which the tnterfeaing 
arches were ftruck, for finding the perpendicular 
