and Miscellaneous Implements at Hull. 
641 
it, the wheel being worlted by an endless hand-rope (m), and the machine is 
supported in a suitable frame with a suspending hook (n) at the top, to attach 
to a beam or other support. 
Headers conversant vvitli mechanics will excuse our iiointing out to others 
that the phiion d, being loose upon the eccentric, does not act as an ordinary 
driving-wheel, but its teeth merely act as wedges, bringing the space between 
the teeth of one of the dished wheels exactly opposite a corresponding space 
in the other wheel. Thus, in Fig. 19 the only spaces that exactly correspond 
are those numbered 1 on each wheel : space 23, on wheel b is now opposite, 
not to space 23 on wheel a, but to the tooth between spaces 22 and 23 on a. 
Half a revolution of d will gradually wedge the teeth apart till 23 is opposite 
the other 23 ; when the revolution of D is complete, space 44 on b will be 
opposite 1 on a. 
The differential power is thus obtained by the gradual displacement of 
the odd tooth in the revolution of the pinion. The discs are perfectly free 
to move either way round in the frame, but the weij^ht on h coming half on the 
right side and half on the left perfectly balances the block and keeps the lifting- 
chains plumb and fair imder its centre. One revolution of the spocket-wheel 
(l) and the pinion (d) will thus cause the disc-wheel (a) to turn over from 
left to right y^ths of an inch, and the disc-wheel (b) will revolve over in the 
opposite direction from right to left an equal distance ; the sum of these dis- 
tances equals (1| inches) the pitch of the teeth on the disc-wheels. The 
diameter of the chain-wheels (f and g) is only half that of the disc-wheels 
(a and b), hence the hook (h) will only rise '34 inch for each revolution of 
the spocket-wheel (l), and to obtain this small rise the hand-rope must be 
moved a space of 6 feet 3 inches. The pulley exhibited is intended to enable 
two men to raise a weight of 7i tons; from the above calculation they would 
have to pull the hand-rope 221 feet to raise the 74 tons 1 foot. The theoretical 
force required would be a pull of 76 lbs. plus the force needed to overcome 
friction. The actual force required was not ascertained by direct experiment, 
but friction would probably cause the total force to be very little, if at all, under 
112 lbs., and two men of average strength might in that case raise the weight 
some 6 feet per hour. 
A second hook could be attached at k, so that a fresh load might be raised 
by reversing the pull of the hand-rope without first lowering the chain. One 
advantage of this system is that the wedge-action of the pinion-teeth prevents 
the weight from running down when left half-raised. 
The prices, of course, vary for blocks of different power ; that of the 7^ ton 
block is 12?. 10s., and the chain costs 5s. per foot, including the hook (h). 
