268 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



the risk incurred in offering a theory which can at once be 

 refuted by a single stroke of the pen. He divides the 

 problem into two parts the mechanical oj^eration of mov- 

 ing the plate to be ruled over given and equal spaces, and 

 the operation of producing on glass lines of varying degrees 

 of fineness. After stating the difficulties in the use of a 

 screw with divided micrometer head, and the errors to which 

 such a screw, made and used with greatest care, is liable, he 

 concludes that the periodicity always to be observed in 

 these errors is due not to the screw itself, but to the mount- 

 ing of the screw ; the system of errors changing, when the 

 same screw was employed, by slight movements of parts 

 against which the screw worked ; in fact, that these errors 

 were due to unequal friction in the movements of the machine. 

 The remedy proposed was to make the ways over which the 

 gravity slide that carries the plate moves adjustable instead 

 of fixed, and a nut at least half the length of the screw, four 

 inches, instead of one, as at first. The gravity slide, having 

 a hollow cylinder firmly attached to the under side, was 

 placed on the ways, and the screw and its nut, passing 

 through the cylinder, were adjusted in position, and then, the 

 slide being firmly clamped on the ways, the space between 

 the nut and the cylinder was filled in with plaster of Paris; 

 it was thus in perfect adjustment for one position of the 

 gravity slide, and any deviation in a horizontal direction could 

 be corrected by means of the adjustable ways. The next 

 operation is to space the lines accurately. For this purpose 

 Mr. Rogers employed a very ingenious arrangement. A 

 rigid arm, two feet in length, was placed so as to vibrate 

 upon a shaft, set exactly in a line with the precision screw. 

 At one end an electric magnet, fitted to the curvature of the 

 head of the screw, was attached by light pivots in such a w^ay 

 as to give parallel motion with respect to the arm ; the outer 

 portion of the head of the screw was a rim of soft iron, oper- 

 ating as an armature. The other end of the arm worked be- 

 tween two stops, one of which was adjustable. When the 

 circuit is completed, the magnet is attached to the head of 

 the screw, and, by the movement of the arm from one stop 

 to the other, is carried over a given space ; the circuit being- 

 broken, the arm during the reverse movement carries the 

 maornet without disturbins: the screw. In order to eruard. 



