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which, from the nature of its action, has been termed the 
" accumulator." Its object is to act as a magazine for so 
much of the force of the engine as is given out during the 
time when the supply exceeds the demand for it, as when a 
part of the machinery in connection with the accumulator 
is at rest. This, it is obvious, must very frequently be the 
case where that machinery consists of cranes or similar 
appliances. The accumulator varies in form, according to 
the nature of the work to be performed by it ; when it is 
probable that the whole of the machinery will be at work for 
the greater part of the day — as, for instance, in a set of 
cranes attached to a bustling dock establishment — the engine 
is of a sufficient size to supply the water as fast, or nearly 
so, as it may be required. If, however, there are portions 
of the machinery which are at rest the greater part of the 
day, as in the case of the dock-gate apparatus at Grimsby, 
the engine is of the size requisite to maintain in action only 
that portion of the machinery which is likely to be very 
frequently in motion, while the remainder can be worked 
from the accumulator when necessary. In the former case, 
the accumulator consists of a strong vertical cylinder, truly 
turned on the outside, over which is fitted, by means of a 
stuffing-box, a second and inverted cylinder, which is loaded 
with weights of iron or masonry to the required pressure per 
square inch. This cylinder, with its load, is supported and 
guided by means of a suitable framing. The force-pipes 
of the pumps being in immediate connexion with this appara- 
tus, supposing no machinery at work, the outer cylinder will 
be driven upwards at every stroke of the engine, until it 
reaches the top of the framing, when a safety-valve opens 
and permits the escape of the water subsequently introduced. 
The hydrostatic machinery receives its supply from this 
accumulator by means of a main carried underground in the 
ordinary manner. In the second case mentioned above, viz., 
