in Buling his Test-Plates. By William A. Rogers, 81 
a slight movement was given to the screws holding the plate against 
which the precision screw works as a shoulder. The sign of the 
errors was again changed, but their magnitude was very much 
reduced, amounting at the maximum to about ^^iTy?}- of an inch. 
This system of errors also remained, as long as no further changes 
were made. 
Having definitely found by these and several other similar 
experiments that the periodicity was not due to the precision screw 
itself, but to the constrained motion caused by unequal friction 
between the nut, the screw, and on the ways on which the gravity 
slide, which carries the plate to be ruled, is moved, I addressed 
myself to the task of removing as far as possible this source of error. 
While I have not succeeded with entire satisfaction, the errors of a 
periodic character have been so much reduced that those which still 
remain give no serious trouble. By a device to be presently 
described, these residuals are overcome by an automatic movement 
connected with the screw itself. Omitting an account of many 
fruitless trials, I describe the following permanent changes which 
were finally made. 
(a) The ways over which the gravity slide moves, one of which 
was at first A shaped, and the other plane, and both of which were 
permanently fixed, were both made A shaped and both movable. 
The ends nearest the point where the bearing of the shaft of the 
screw works against its shoulder, were pivoted. The other ends 
were made adjustable with set screws. The precision screw being 
set in its normal position, and attached to the slide by its nut, the 
ways are set parallel with the screw by the motion of the slide 
upon them. 
(h) The nut, which at first was only about one inch long, was 
made four inches in length, being one-half the length of the screw. 
About equally good results were obtained with a lead and a brass 
nut. The lead nut is much the more difficult to make, as a tap 
cannot be used. Even when it was cut with a chaser on the lathe, 
it was found impossible to get a smooth thread until the very 
simple remedy of keeping the interior wet with a strong soap lye 
was tried. 
The nut having been fitted to the screw, the threads were 
reduced to a homogeneous system, and at the same time polished, 
by grinding with the finest emery. It should be remarked that the 
screw was originally finished in this way, using coarser emery at 
first. The rule adopted was to grind the screw till all tremor per- 
ceptible to the touch in the passage of the nut over the entire 
length of the screw disappeared. 
(c) In order to set the screw parallel to the ways in a vertical 
direction, a hollow cylinder was firmly attached to the under side 
of the gravity slide. The screw, with the nut upon it, and passing 
