394 -An, Account of the Measurement 
themselves to the sides of the conical axis, but do not retard the 
free motion of it, when the telescope is moved in the direction 
of the plane of its arch. These wheels, two on each lever, sup- 
port the axis near the junction of the telescopic tube ; and, as a 
few ounces only are by these means suffered to press on the 
pivots, no bending takes place in the cones. 
From the middle of the two uppermost horizontal cylinders, 
which unite the sides of the interior frame at top, and which 
receive, with the two beneath them, the respective Y plates, 
arises a small but substantial apparatus of metal, embracing a 
hollow brass cylinder, of five inches in diameter, and about three 
deep, which passes up into an opening in the upper part of the 
external mahogany frame. This cylinder, with its corresponding 
stand, are sustained, without any sort of shake, by a helical 
spring. This mode, with that of supporting the azimuth circle 
below, are so well managed, that when the instrument is pro- 
perly adjusted for observation, the axis of the sector continues 
perfectly horizontal, in every position of the frame. 
There is likewise a very convenient method of sustaining the 
sectorial tube in any required position for observation. Across 
the interior frame, about the height of the graduated arch, run 
two long brass axles, with two wheels on each, one precisely in 
the middle of the axle, (and consequently in the same plane 
with the line vertically cutting the middle of the telescope,) and 
the other close to the pinion at the end of the axle. From a 
steel pin, something peculiar in its construction, situated near 
the end of the telescope, proceeds a string, which is wound eight 
or ten turns round the pulley. Attached to the inside of the 
interior frame, and just above the wheels nearest to the end of 
the axle, is another pulley, over which, passing into a long and 
