394 
Dr, Herschel's Description of a 
eastern rubbing-plate, is a small square bar with a loop upon 
it, which is adjustable, so that it may be occasionally brought 
a little nearer to the mouth of the telescope, or removed 
farther from it. The end of the polar distance cord is fastened 
to the loop upon this bar, where it remains when the polar 
distance clock is not in use. By this means the weight which 
stretches it in all its length from the telescope to where it is 
suspended in the clock-case, is kept always equally exerted, 
and no relaxation of the cord, which ought to be avoided, can 
take place. 
When the polar distance clock is to be used, the cord is 
lifted into the pulley of the slider at m, and now goes from 
thence to its destination as before. The right ascension roller 
resting against the rubbing plate, the pulley of the slider is 
near at hand, and the cord may easily be lifted into it. The 
handle o is now to be turned till the cord, which goes from 
the loop at the telescope to the pulley upon the slider comes 
to cover a certain white line or mark upon the side of the 
tube. This line when it is first made, must be placed so as 
to be vertical when the radius of motion of the loop is a little 
more than one degree of elevation above the horizon. 
The theory of this arrangement is, that when a motion in 
polar distance takes place, the tangent and the arch may be 
looked upon as equal for a few degrees, in a mechanism which 
aims only at minutes. And, indeed, as far as two degrees and 
twenty minutes, when the motion is taken equally both ways 
of the adjusting point, the deviation from truth will not even 
ampunt to quite one second. 
The cord from the pulley of the polar distance machine passes 
