430 Sir J. F. W. Herschel on a Machine 



off on the vertical scale at /. The zeros of both readings corre- 

 spond to the horizontal position of the line CB, which position 

 having been once well ascertained and verified, may be afterwards 

 at any time recovered by a small level PS screwed on the cross- 

 piece PQ, and adjusted accordingly. Both zeros are adjustable — 

 that of the index circle by the stiff-friction motion of the index- 

 arm CH, and that of the scale, first, coarsely by loosening the 

 excentric wheel on its center, and winding or unwinding the thread 

 on its circumference, secondly, more delicately by screwing or 

 unscrewing the peg p to which its end is fastened. 



(8.) The division of the vertical scale must be into 360 equal 

 parts or degrees, the whole length from to 360 being that of 

 the circumference of a circle whose radius is 1. This length may 

 be determined (better than by any attempt to measure the radius) 

 by turning the axis C round through one complete revolution, and 

 noting by temporary marks or dots on the scale the points inter- 

 sected on the fiducial line, by the horizontal straight edge IJ at the 

 two extremities of its motion. The interval between these dots 

 is the value of 360 parts required, and must be subdivided ac- 

 cordingly. It is evident that whatever be the value of e in the 

 equation u-\ c . sin u = A an increase of 360" in u must correspond 

 to 360° increase in A, so that the accuracy of this process is, theo- 

 retically speaking, independent of the position of B on the slider — 

 but practically it is preferable, before executing this most essential 

 of all the preliminary operations, to bring the center of the ex- 

 centric as nearly as possible to coincidence with the axis C, because 

 in that position any slight deviation from horizontality in the line 

 I J will not influence the result. 



(9.) The perfect horizontality of this line is however of ma- 

 terial import to the correct performance of the machine. It is 



