W. A. Rogers—New Diffraction Ruling Engine. 5T 
(b.) Usually the aliquot part of a pressor of the screw is, 
during the process of ruling, secured by means of a pawl act- 
ing upon the teeth cut upon the index. In “this case one is 
limited to certain fixed ares of revolution. In the Waltham 
machine a magnet arm 24 inches long rests upon the axis of the 
ser of the arm works between two movable stops. 
A magnet fitted to the curvature of the index is attached to the 
other end. Another magnet is attached permanently to the bed 
of the machine beneath the index. During the upward move- 
ment of the magnet-arm, the first magnet becomes firmly 
attached to the index, carrying it forward, an are of revolution 
depending on the distance between the sto ops. During the 
downward movement of the arm the index is held in position 
by the second magne 
It will be seen that the are of revolution may be made to 
vary at will between zero and a value limited by the greatest 
distance between the stops, which in this case corresponds to a 
oO of a little over one thousandth of an inch in the screw- 
The index of the screw regulating the distance between the 
stops reads directly to about one millionth of an inch ex- 
pres in the corresponding motion upon the ruling screw. 
ince these divisions can be estimated to tenths, as small a 
movement as one ten-millionth of an inch can be given to the 
screw-plate with entire certainty as jar as the mechanical indica- 
tions of this degree of precision are con 
c.) In order to provide for the sovitel ination of whatever 
errors might be found to poe in the screw in actual use, the 
following devices were adopted :— 
(1.) Instead of allowing ao magnet-arm to fall upon a fixed 
stop, it falls upon a circular templet, having a motion in revo- 
lution simultaneous with that of the index of the screw. 
mental corrections will be found necessa 
