DUPLEX OR EITHER-SIDE LATHE. 145 



Hence, instead of having to remove the point and 

 the smaller corundum wheel in order to change the 

 larger one for a finer or a coarser, a work which 

 involves some loss of time, and what is perhaps still 

 worse, considerable wear and tear to the fingers, all 

 that is necessary is to withdraw a small pin that 

 passes through the boss in the centre of the bridge, 

 and having turned the headstock so as to bring the 

 other wheels into use to replace the pin. 



It will be observed, that in this operation of 

 turning or reversing the headstock, the catgut band 

 of necessity becomes crossed below the pulley. This 

 led me to adopt the suspension principle to the 

 headstock. 



It is obvious, moreover, that if the distance 

 between the pulley in the headstock and the driving- 

 wheel remained fixed, a longer band would be 

 required when it was crossed than when it passed 

 directly or uncrossed. In order to meet this 

 difficulty, I have made the headstock to turn in the 

 boss upon a double threaded screw of one inch 

 pitch, by which I get, in the half re volution required, 

 an elevation or depression, as the case may be, of 

 half an inch, which corresponds exactly with the 

 range which experiment showed to be necessary 

 between the pulley and the driving-wheel, upon 

 crossing or uncrossing the band. By this, con- 

 trivance the catgut band is maintained of an uniform 

 tightness in every position of the headstock, and is 

 thus saved from being broken by undue tension, on 



