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28 MODERN MAGIC LANTERNS. COMPRESSED GASES. 29
pressure gauge is shown at Fig. 21, and is usually graduated
with two scales, one reading from 0 to 110, or thereabouts,
Fig. 21. PRESSURE GAUGE.
and the other from 0 to 10, the 120 of the one scale corresponding
with the 10 of the other. The smaller divisions
are atmospheres (an atmosphere is taken as a pressure
of 15 lb. to the square inch) and at a pressure of 120 atmospheres
a " 10-foot " bottle contains 10 feet of gas, which is
read off direct on the other scale. If the cylinder is a 20foot
or 40-foot, the number of cubic feet indicated on the
gauge must be multiplied by 2 or 4 respectively ; if a
12-foot, by 11, and so on, multiplying in each case by the
number of times 10 feet are contained in the nominal
capacity of the charged cylinder. In this way the contents
of the cylinder can be gauged. Those who do not care to
incur the expense of a gauge can ascertain roughly how
much gas the cylinder contains by weighing it. Inasmuch
as 12 cubic feet of oxygen gas weigh about a pound
avoirdupois, if the weight of the cylinder empty is known,
the difference in ounces between that weight and its weight
when it contains an unknown amount of oxygen must be
multiplied by 3 and divided by 4, the result being the
number of cubic feet of oxygen it contains.
For example : A cylinder which when empty weighs
25 lb. 14 oz. is found to contain a certain amount of oxygen
making it weigh 27 lb. 81 oz. It is required to know
how much of the gas it contains. On subtracting 25 lb.
14 oz. from 271b. 8 ozs., we get 251 oz. Multiplying
251 by 3 and dividing by 4, the result is 194the number
of cubic feet of gas the cylinder contains. This may be
taken as a rough guide, but too much dependence should
not be placed on information got by means of an operation
where a very slight mistake may greatly mislead. In the
case of coal gas, no useful indication of the amount of the
contents of a cylinder can be got by weighing.
Gauges, like regulators, must not be used indiscriminately
for oxygen and hydrogen, and on turning the gas
into them it must be done gently and by degrees, on no
account with anything like a jerk. With this end in view,
indeed, the general custom now is almost to close the
entrance to the gauge, only leaving the minutest hole
through which the gas can but gradually force its way, no
matter how suddenly it may have been turned on.
With a view to prevent the filling of a bottle which might
contain some hydrogen with oxygen, or vice vein& and the