the ST ngonometrical Operation . 1 8 9 
placed, having a fmall crofs made of two thin bits of wood 
fitted to the infide of the bafon, and lying very near the fur- 
face of the quickfilver, in fuch a manner as to make the center 
of the crofs co-incide with the interfe&ion on the brafs under- 
neath. A fmall perfpedtive glafs being then fixed in a move- 
able board under the center of the inftrument, this was made 
to Hide at right angjes to itfelf in the diredtion of the meridian 
and that of the axis of the tranfit, until the center of the crofs co- 
incided with the axis of vifion in looking downwards. The 
board being there faftened, and the perfpedtive removed, the in- 
terfedtion of filk threads ftretched acrofsthe board, marked very 
accurately the point correfponding with the center of the tran- 
fit, over which the center of our inftrument was brought by 
the help of the plummet. The fecond method was ftill more 
diredt. Dr. Maskelyne had an objedt glafs prepared for his 
tranfit telefcope of a focus fuited to the vertical height of the 
ftand of our inftrument above it. This glafs being applied to 
the tranfit, and the aperture contradled by a piece of pafte-* 
board with a circular hole in the middle, a very fmall pin-hole 
being likewife made in the board at top, the fame was grar 
dually moved by diredtions from the obferver below, looking 
through the telefcope in its vertical pofition, until the pin-hole 
nicely co-incided with the axis of vifion. The inftrument 
was then brought as before, by the help of the plummet, 
exadtly over the pin-hole. In this manner, which was that 
adhered to, no doubt remained of more than about T A„th part 
of an inch, with refpedt to the center of the inftrument being 
in the interfedtion of two vertical planes pafling through the 
axis of vifion and that of motion of the tranfit underneath* 
After having remained a week, the, co-incidence of the pin- 
1 hole 
