112
--- _p_ π _m_ _x_².
800
For example, let the diameter of the gas-holder be 30 feet, the height
15 (the contents in cubic feet will be 10,597), _p_ = 4 pounds; then the
counterweight for a height of an inch and a half of water pressure =
3532 pounds; the weight of the chain for a length of 15 feet = 395
pounds. Were no counterweight employed, so that the gas-holder pressed
with its whole weight upon the gas, then the height of the equivalent
column of water in its highest position = 2·56 inches; and in its
lowest, 2·33. The counterweight may hence be lessened at pleasure, if
the height of the pressing water-column _n_ be increased. The weight of
the equalising or compensating portion of the chain remains the same.
When _n_ = 2 inches, for instance, the counterweight = 1886 pounds.
The velocity with which the gas passes along the mains for supplying the
various jets of light, may be further regulated by opening the main-cock
or slide-valve in a greater or less degree.
Gasometers whose height is greater than their semi-diameter, are not
only more costly in the construction, but require heavier counterweights
and equilibration chains.
The above estimate is made on the supposition of the gas in the
gas-holder being of the same specific gravity as the atmospherical air,
which would be nearly true with regard to oil gas under the ordinary
pressure. But coal gas, whose specific gravity may be taken on an
average at about 0·5, exercises a buoyancy upon the top of the
gas-holder, which of course diminishes its absolute weight. Supposing
the cubic foot of gas to be = 0·0364 pounds, the buoyancy will be =
0·0364 π _x_³ pounds; a quantity which deserves to be taken into account
for large gasometers. Hence,