316 
one on each side of the centre of motion of the crane-frame. 
The rams of these cylinders likewise carry pulleys, which 
act upon a chain, one end of which is fastened to each 
cylinder, in such a manner that as one ram moves out of its 
cylinder and shortens its chain, the other enters its cylinder 
and gives out an equal length, so as to produce a movement 
of the chain in either direction, according* as one or other ram 
is exposed to the pressure of the water, while the opposite 
one is relieved from it. The motion of the chain is commu- 
nicated to the crane-frame by a pulley fixed upon the latter, 
into which the links of the chain are made to fit. The crane- 
chain is overhauled by means of a fourth hydrostatic cylinder 
of small diameter, but having its stroke equal to that of the 
hoisting-ram. This cylinder contains a ram which is always 
exposed to the pressure, and which acts in a direction exactly 
opposite to that of the hoisting-ram, the latter being made of 
a sufficient size to overcome the reaction of the over-hauling 
ram', as well as friction and the weight of the load. When 
the load has been hoisted up, and is to be lowered again, or 
when the chain is to be returned after delivering its load, all 
that is necessary is to cut off the pressure from the hoisting 
cylinder, and to provide an escape from it. This allows of 
the immediate action of the over-hauling ram, which drives' 
the hoisting-ram back into the cylinder, and thus returns the 
crane-chain to its original position. It is obvious that the 
load can be maintained suspended at any height when con- 
venient to do so, by merely cutting off the exhaust of the 
hoisting-cylinder. 
Several ingenious contrivances are applied for removing 
any portions of rust, &c, from the water, before it is admitted 
to the valves or cylinders ; for preventing the chain of the 
turning-barrels from falling off the pulleys, in case of the 
crane being turned any distance by hand ; and for other 
purposes. The framing of the cranes at Grimsby is some- 
