[ 3 6r 3 
being; meafured — — th of an inch; whence the angle 
of deviation of the wires, from a right angle, is 21 
minutes. But, by a mean of 1 1 trials, the quantity 
of the deviation came out 28'' JL, the extreme refults 
being 21' and 36'. This is the deviation of the 
fouth part of the middle horary wire, from a perpendi- 
cularity, to the directing wire towards the eaft, in the 
obfervations at St. Helena ; a ftar, that paffed fouth 
of the centre of the telefcope, coming to the ho- 
rary wire too foon, and a ftar that paffed north of the 
centre coming later to the horary wire than it ought 
to do. 
In order to determine whether the two other ho- 
rary wires were parallel to the middle one, or, if 
not, what angle they made with it, I compared the 
tranfit of 13 ftars acrofs the three horary wires, with 
thofe of as many more ftars differing confiderably 
in declination from the former, all obferved at St. 
Helena; and from the differences of right afcenfion 
at the feveral wires, after making an allowance for 
the convergence of the meridians, which however is 
not 2 I found the fouth part of the firft or eaftern 
wire to deviate from a parallelifm with the middle 
one towards the weft by </, 8, and the weftern wire 
to deviate towards the eaft by 5^,4 ; hence it appears 
that the fouth parts of the eaftern, middle, and weftern 
horary wire differed from a perpendicularity to the di- 
recting wire towards the eaft, in the obfervation of 
St. Helena, by 18^7, 28' .1, and 34', the mean devi- 
ation of all the three being 27' or only 1 4- different 
from that of the middle horary wire. This quan- 
tity of the mean deviation of the wires is alfo con- 
Vol. L1V. A a a firmed 
