All these
defects, including that of the necessity for very substantial foundations
under the holder to support its enormous weight, may be overcome by
adopting the second method of construction. It is clear that the water in
the centre of the tank is of no use,--all that is needed being a narrow
trough for the bell to work in. Large rising holders are therefore
advantageously built with a tank formed in the shape of an annulus, the
effective breadth of which is not more than 2 or 3 inches, the centre
portion being roofed over so as to prevent escape of gas. The same
principle may be retained with modified details by fitting inside a plain
cylindrical tank a "dummy" or smaller cylinder, closed by a flat or
curved top and fastened water- and air-tight to the bottom of the main
vessel. The construction of annular tanks or the insertion of a "dummy"
may be attended with difficulty if the tank is wholly or partly sunk
below the ground level, owing to the lifting force of water in the
surrounding soil. Where a steel tank is sunk, or a masonry tank is
constructed, regard must be paid, both in the design of the tank and in
the manner of construction, to the level of the underground water in the
neighbourhood, as in certain cases special precautions will be needed to
avoid trouble from the pressure of the water on the outside of the tank
until it is balanced by the pressure of the water with which the tank is
filled.
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