315 
when very large demands are likely to be occasionally made 
on the accumulator, it takes the form of a reservoir of 
sufficient size, placed at such an elevation as to give the 
required pressure. 
The principal working parts of the hydrostatic crane may 
be briefly described in a sufficiently clear manner, without 
the help of a diagram. They are required to effect the 
following objects: — First, to hoist the load to a certain 
distance ; secondly, to give it a lateral motion, usually per- 
formed in a circular direction ; and thirdly, the lowering or 
" over-hauling" of the chain with or without the load. The 
first movement, or the hoisting of the load, is effected by 
means of a ram or plunger working water-tight in a cylinder, 
fixed in any convenient position, which carries at its exposed 
end one or more pulleys, over which the crane-chain passes. 
One end of this chain is shackled to the cylinder, while 
the other is furnished with the usual means of attaching the 
load. The effect of a stroke of the plunger is to shorten 
the crane-chain by twice the length of the stroke, or by 
more, according to the number of pulleys — each pulley 
shortening the chain twice the length of the stroke. Hence, 
with three pulleys, and a plunger having a stroke of 10 feet, 
the chain would be shortened 60 feet, or the load attached to 
the end of it would be raised 60 feet. In this case, owino- to 
the mechanical disadvantage at which the ram acts, the 
weight of the load raised must be less than one-sixth of the 
stress applied to the plunger, or the force to be applied must 
be more than six times as great as the weight of the load to 
be raised. This multiplication of force, however, is readily 
attained by increasing the area of the end of the plunger, at 
a corresponding expense of water and engine-power. The 
lateral motion of the load is effected by swinging the crane 
round, as in the ordinary manual-power machine. This is ac- 
complished by means of two other hydrostatic cylinders, fixed 
y 2 
