of an Arc of the Meridian. 395 
narrow wooden compartment, is thrown a string, having a hook 
and a proper apparatus for receiving the moveable weights. 
The other end of this string is fastened to the pulley close to the 
axle, and gives motion to the telescope, or retains it in equilibrio, 
according to the arrangement of the two sets of weights, which 
consist of fifteen pieces of brass. By these means, all injurious 
pressure is taken off the point of the micrometer-screw, against 
which the telescope may be made to bear, with any required 
degree of force. 
To cause the string passing over the middle of the pulley to 
draw in the exact direction of the limb’s plane, Mr. Ramsden 
placed four small friction-wheels close to the eye end of the 
telescope, two on each side, and between each pair of wheels a 
steel pin, made like a T, with a hook at the end, to receive a 
string. This pin, where it applies to the wheels, is something 
in shape like a double cone, and is passed behind them. It 
always, from its construction, assumes the same position with 
regard to the friction-wheels ; from which circumstance, the 
sustaining string is ever found in the plane passing through the 
centre of the telescope and the middle of the pulley. 
The micrometer-screw, for measuring minutes and seconds, 
performs its operations in the usual way : it is moved backwards 
or forwards on a brass arch, parallel to the limb of the sector, 
and placed against the mahogany frame behind. To this arch 
the apparatus carrying the micrometer-screw is clamped ; and 
it is adjusted, or brought parallel to the limb, by screws, so that 
the point of the micrometer-screw always bears exactly on the 
same part of the polished steel head, at the end of the sector. 
The principal wires in the focus of the eye-glass are two, and 
are at right angles to each other. There are, indeed, two others 
