624 



Proccedinrjs of the Royal Physical Society. 



and made by Hume, Edinburgh, is a modification of the worm 

 screw form. On the spindle driven by the pulleys (/) are two 

 worm threads, a coarse one {d) and a finer one {e). On the drum 

 spindle are two wheels (c), one with large, the other with 

 smaller cogs, corresponding in size with the worms on the 

 different sections of the spindle. The upper wheel with the 

 fine teeth is driven by the fine worm which is placed nearer 

 the driving pulley ; the lower wheel, smaller in diameter, is 

 driven by the coarse worm, placed at the end of the screw. 

 In order to obtain the different rates of speed from these. 



Fig. 4. 

 {a) Fixed pulley. 

 (&) Kunning pulley 

 (c) Sliding gear. 



Fig. 5. 

 {a) Fast drum. 

 {h) Loose pulle)^ 

 (c) Clutch. 



all that is necessary is to raise or lower the driving spindle 

 and slightly alter its angle, so as to bring the one or other 

 worm in contact with its special wheel. The drum is not 

 driven directly from the shaft but from a clutch pulley (Fig. 4), 

 which is constructed from plans drawn by Dr Milne Murray. 

 It consists of a large pulley (a) fixed to a spindle, and a pulley 

 or cone of pulleys (h) running loose on the same spindle. On 

 the inner surface of the loose pulley is a pin with a conical 

 head, and on the fixed pulley is a ring of bevelled holes, 

 into one of which this pin may fall when the loose pulley 

 is brought up. This does well when the speed is not too 

 high, but for higher speeds I prefer a friction pulley, in 



