TRANSACTIONS OF SECTION A. 



469 



To effect this tlie following plan has been adopted with success. 



On one end of the screw a wheel, W, six inches in diameter, with a smooth 

 edge, is fixed, and in the continuation of the axis of the screw behind the dead- 

 centre, L, the hollow axis, D, held in rigid bearings bolted on the bed of the 

 machine, carries the iron casting, N. N contains what may be described as an 

 artificial hand, of which the finger and thumb, c c', when tightened, close on a 

 diameter of the wheel W. The end of the lever, N', is actuated, through a 

 connecting rod, by an adjustable crank, Q, fig. 1, and suitable mechanism removes 

 finger and thumb from the periphery of the wheel during one-half of the revo- 

 lution of the crank, and tightens them on it during the other half. This causes 

 the screw to turn round by a definite amount at each complete revolution of the 

 crank. 



He had now to mention the means employed to make this rotation independent 



Fig. 3. 



of the various errors of workmanship which are sure to be present in such a con- 

 trivance. 



In the first place, no strain on the dead-centre support of the screw should 

 result from the pressure of the ' hand,' nor should any slight eccentricity of the 

 wheel affect the amount of the rotation. This is ensured by the arrangement of 

 the levers which open and close the finger, shown in fig. 3. The inner end of the 

 levers, c c', terminate in front of the hollow axis, D. A rod, K, passing through 

 D carries a cross head, /j, capable of pivoting through a small arc about l^ in 

 the plane of the levers, c, c'. Links, ^ l^, connect the ends of the cross-head 

 Nvith those of the levers ; thus the backwards or forwards movement of the rod, R. 

 in the hollow axis settles the distance between the jaws of c, c', but not their 

 absolute lateral position, and the equality of the lengths of the levers and arms of 



