SECT. II.]
PROPERTIES OF STEAM.
101
169. When C A is the height of a uniform
atmosphere, C D the height of the chimney, and
D E the quantity the air expands by the heat it
receives in passing through the fire, the height E D,
or F G its equal, represents the height of the co
lumn of air which produces the motion, and the
velocity will be that a heavy body would acquire
by falling through the height F G. If the whole
of C A were empty, then B H, the height of the
atmosphere, would be the height through which the
body must fall to acquire the velocity with which
the air would move into the tube, provided it suf
fered no contraction at the entrance; but such a
contraction is well known to take place in air as
well as in water.
170. When this is applied to a chimney, the smoke being sometimes of a
density different from common air at the same pressure and temperature, the same
excess of temperature will produce a greater or less effect in proportion as it is of
less or greater density than common air. This will be found by subtracting from
the expansion the specific gravity of the smoke or vapour, that of air of the same
temperature and pressure being unity. Or it may be done by an allowance of a
portion of the temperature for the difference of density : either method gives the
same result when properly calculated. In this case I intend to adopt the former
method. The latter is followed in my book £ On Warming and Ventilating
Buildings.’ 1
171. Let h be the height in feet from the place where the flue enters to the
top of the chimney; e = the bulk to which one foot of air increases by the change
Fig. 14.
A. -B
1 The principles of calculation followed, both in this and in the work referred to, are perfectly
identical with those employed by Mr. Gilbert, in an excellent paper on the subject in the
‘Quarterly Journal of Science,’ vol. xiii. p. 113 : but the notation and methods of managing the
processes are different; and Mr. Gilbert’s mode of calculating the expansion does not afford quite
satisfactory results : besides, he makes no allowances for the contractions and loss of force in
curvilinear motion. I mention the circumstance, because some people compare and criticise, and
imagine those things to be different which are in reality identical, as may easily be shown by
putting both in the same notation, and reducing by the rules of algebra. The great object of a
practical analyst is to render the final equation as easy of application as possible. As to those who
question principles, it is rather unfortunate for them to question those established principles of
pneumatics which are confirmed by experiment. It is only when theory and experiment do not
agree, that the principles can be called in question.