State of the Useful Ai is. 267 



this, Mr Whitworth has placed four friction-rollers, two bearing 

 on the one side, two on the other side of the thread (I think a 

 four-inch thread), so that, by this combination of rollers, the 

 friction is so much reduced, that a very slight pressure suffices 

 to put the instrument in motion ; the application of the screw 

 gives peculiar facihties for the reversion of the motions. 



The complete turning-lathe contains all the elements of the 

 screwing-engine, so that it is hardly worth while to examine 

 them apart ; indeed I am not aware of any recent improvement 

 in the one which does not apply to the other. 



In using the ordinary turning-lathe, the cutter is held in the 

 hand, and rested against a strong obstacle. By the motion of 

 the hand the cuttinof edge is brou<Tht in contact with the revolv- 

 ing substance, and part of that substance is cutaway ; but whe- 

 ther the cut be circular or not depends altogether on the steadi- 

 ness of the hand. If the lathe-spindle turn very slowly, as in 

 turning large pieces of cut-iron, even the most practised and 

 patient workman would find it impossible to keep the cutter so 

 steady in its place as to present considerable errors in the work ; 

 but if the revolution be very rapid, as in turning soft woods, 

 there is scarcely time given for any important change in the 

 position of the tool. Hence the slide-rest was most readily 

 adopted where large masses were operated on. This instrument 

 retains the tool securely in its place during the revolution, 

 and thus produces work much more precise than could pos- 

 sibly be obtained from the use of the hand-tool. The slides 

 and screws of the slide-rest enable the operator to move the cut- 

 ter, so as to give to the work any required form. If the work 

 is to be cylindric, conical, or flat, he has only to adjust one of 

 the slides to the proper angle in order to obtain the wished for 

 result. But even when the surface wanted is generated by the 

 revolution of a curve, the experienced operator, with a well-ad- 

 justed slide-rest, will produce work scarcely surpassed in sweet- 

 ness of outline by that of the hand-tool. 



The transition from the slide-rest to the self-acting lathe is 

 simple and natural ; we have only to connect the screws of the 

 slide-rest with the spindle of the lathe, so that the motion of the 



