The Account of a 
44 ^ 
are two nuts, one sliding within the other. To the upper end 
of the interior one the micrometer is fixed ; and near the 
lower end are three prongs similar to those above mentioned, 
but something longer. These prongs pass through slits in the 
exterior tube, and are confined between nuts, in the same 
manner as the object-lens. This construction has many ad- 
vantages over that described in the Philosophical Transactions. 
To obviate the necessity of the gold tongue (Phil. Trans. 
Vol. LXXX. p. 147), besides the moveable wire in the field of 
the microscope, there is a second one, which may be consi- 
dered as fixed, having only a small motion for its adjustment. 
When the instrument is adjusted, and the index belonging to 
the micrometer-screw stands at the zero on its circle (the 
moveable wire cutting one of the dots on the limb of the in- 
strument), this fixed wire must be made to bisect the next 
dot ; as by this means it may be perceived at any time, whe- 
ther the relative position of the wire has varied. 
By graduating the limb of the instrument to every ten mi- 
nutes instead of fifteen, we are enabled to measure by the 
micrometer-screw, not only the excess of the measured angle 
above any of the ten minutes, but also its complement to the 
next division on the circle, and thereby to correct any small 
inequality which may happen between the divisions. 
x 
art. hi. Particulars relating to the Operations of the Tear 
1792. 
Although it might have been reasonably supposed, that the 
angles of the triangle King’s Arbour, Hampton Poor House, 
and St. Ann’s Hill, had been observed with sufficient accuracy 
in 1787, yet that this operation might not rest on data afforded 
