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.