SECT. II.]
PROPERTIES OF STEAM.
63
The explanation offered by Mr. Watt himself is not sufficient to account for the
difference, except in the lower temperatures. He supposes the stationary baro
meter must have had its scale placed *2 of an inch too low; and if so, the same
addition would be required to the forces in the preceding table on salt water.
These tables, however, are not selected for minute accuracy, but to show the
important fact, that the force of the steam of water depends on the temperature of
the liquid which produces it, or which is in contact with it. For this they are
sufficiently correct; and it is a circumstance which affects its elastic force both in
the boiler and in the condenser, and is peculiarly interesting to those concerned in
steam vessel engines. The temperatures not being the same, the comparison is not
so easy; but at 180° the force of salt water is 10-85; that of pure water 14-73
inches : at 212° salt w T ater has a force of 22*74 ; pure water 29*56.
95. The experiments made by Professor Robison were tried in a similar
manner; and as a method the same in effect was used by Bettancourt, whose
results agree extremely well with Robison’s, the description of it may be useful.
Professor Robison’s apparatus for determining
the force of steam.—This apparatus, in the first
trials, consisted of a small digester of copper, A B
CD, in the figure : the top had a thermometer
inserted through the centre, and a loaded valve at
V ; and a third hole for inserting a barometer tube
S G F, to ascertain the force at lower temperatures
than 212°. The force at the higher temperatures
than 212° was measured by the steelyard on the
valve, a plug being inserted in the place of the tube
S G F; but the results with the valve were irre
gular and unsatisfactory. Hence, the glass tube
M N K, having a cistern L for mercury, was adapted
to the hole in the digester; and instead of mea
suring the force by the valve, it was measured by
the ascent of the mercury in the tube M N. The
digester was heated by a lamp.
To determine the pressure at temperatures below 212°, the tube SGF was
inserted as in the figure, and a basin of mercury provided at F. The lamp being-
applied, the water in the digester produced steam till it issued at both the valve
and the pipe F, so as to expel the air : the lamp being removed, and both the
valve and tube being closed, the latter by immersing it in the mercury, the mer
cury rose in the tube F G as the apparatus cooled, and the heights corresponding
to different temperatures were noted : like observations were made as it re-heated.
Fig. 10.