OF ARTS AND SCIENCES. 245 



(o) The ways over wliit-li the f:;r:ivity sli<le moves, one of which was 

 at first A shaped, and the other plane, and hotli of which were per- 

 manently fixed, were both made /\ shaped and hoth movable. The 

 ends nearest the point where the bearing of the siiaft 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 nor- 

 mal position, and attaciied to the slide by its nut, the ways are set 

 parallel with the screw by the motion of the slide upon them. 



(b) 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 perceptible to the touch 

 in the passage of the nut over the entire length of the screw disap- 

 peared. 



(e) 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 jiassing 

 through this cylinder was first set in position. The gravity slide 

 having been firmly clamped down upon the ways, the open space sur- 

 rounding the nut was then filled with plaster of Paris. 



In this way the screw is set in perfect adjustment for one position 

 of the gravity slide. Practically, it is found to be in good adjustment 

 for every position upon the ways. But any slight deviation from 

 adjustment in a horizontal direction is corrected by means of the 

 adjustable ways, while that in the vertical direction is for the most 

 part overcome by leaving one end of the precision screw fi-ee. 



Good results have also been obtained by using a " free nut." In 

 this case nice adjustments are unnecessary, as the nut moves freely 

 upon the screw, pushing the gravity slide before it. If this arrange- 

 ment is adopted, care should be taken that the nut, if not symmetrical 

 with respect to the screw, should fall freely in the direction of gravity, 

 and bear at every point throughout its whole length against whatever 

 holds it in position while the screw is in action. The most serious 



