BOILER ACCESSORIES 129
heating the water and the steam in the boiler is concerned. The
standard temperature at which the hot gases are usually delivered to the
chimney is in the neighbourhood of 600°. There is a limiting value
to the temperature at which the hot gases are delivered to the chimney,
for the following reason. Though the density of the gases decreases
in direct proportion to the absolute temperature, the velocity at which
they pass through the chimney increases as the square root of the
absolute temperature, and the limiting value-the economic tempera-
ture is found to be 550° F. above the temperature of the atmosphere.
As the average in this country is in the neighbourhood of 60°, this
means that the limiting temperature is in the neighbourhood of
600° F. It is the velocity at which the gases pass up the chimney,
which controls the intensity of the draught. The force required to
drive the air through the boiler furnace, and to drive the hot gases.
through the boiler flues and up the chimney, because some force is
expended even in overcoming the friction of the hot gases on the
walls of the chimney, both in mines and in boiler furnaces, is
measured by a special apparatus, known as a water-gauge, the force
being described as so many inches of water-gauge. One inch of
water-gauge corresponds to 0.578 oz. pressure per square inch, and is
arrived at in the following manner. The weight of a cubic foot of
water is 62.355 lbs., at 62° F., and consequently that is the pressure
exerted by a cubic foot of water upon an area of one square foot.
The weight of a mass of water I square foot in area and 1 inch in
62.355
depth is
= 5.197 lbs., and the weight of a cubic inch of water
12
will be the weight of this last quantity-5.197 lbs. divided by 144
(the number of square inches in a square foot) = 0.578 oz., and this is
therefore the pressure which 1 square inch of water excrts upon the
square inch base it stands upon, and is the value of the inch water-
gauge.
NOTE. The inch water-gauge must not be confused with the
miner's inch, which is something quite different, and has nothing to
do with pressures.
The water-gauge usually consists of a U-tube, having a scale
representing the difference of level between the two legs, reduced to
inches of water. The two legs are arranged to be open to the two
atmospheres, or the atmosphere and the body of gas, the difference of
pressure between which it is desired to measure. There are different
methods of accomplishing this, rubber tubes slipped over the ends of
the legs of the tube, being a simple and favourite one. One simple
arrangement is, the U-tube is fixed on a piece of board, or in any
convenient position, in one body of gas, say in the atmosphere, and a
short tube is led from the end of one leg, through an aperture
provided for the purpose, in a partition dividing the two bodies of
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