OF EXPANSION OF SUPEEHEATED STEAM. 
593 
taken in a series of descending temperatures, to avoid the influence of steam boiling out 
of the mercury as the temperature rose, and to eliminate the effect of the cohesion of 
the glass on the water, as explained in our previous paper on the Density of Steam. 
Capacity of the Globe. 
Twelve cubic inches of mercury were measured into the globe, and a file-mark made 
on the stem. 
Below the flrst file-mark, at a distance of 14*45 inches, another file-mark was made to 
afibrd a fixed point for ascertaining the correspondence of the upper file-mark with the 
readings on the fixed graduated rod or cathetometer. 
Correction of Readings. 
Let a be the reading on the fixed rod of the level of the column ; b be the reading of 
the lower file-mark on the globe-stem. Then 
-\-b—a=i\ie height of the column of mercury in the globe-stem. 
To correct this for temperature, 7-| inches of mercury, included in the oil-bath and 
its stuffing-box, were corrected for the temperature of the oil, and the remainder for the 
temperature of the atmosphere at the time. 
By deducting the column so corrected from the reading of the barometer at the time, 
the total pressure in the globe is obtained. 
The readings of the thermometer are corrected for the portion out of the oil-bath. 
The pressure of the vapour of mercury is calculated from data supplied with great 
courtesy by M. Eegnault, and embodying the results of unpublished experiments. The 
pressure of this vapour is assumed to be the same as that in a vacuum, as the vapour in 
the globe remains still for a sufficient time (it is believed) for saturation to take place. 
In this view we have been strengthened by M. Regnault’s opinion. 
By deducting the pressure of mercury vapour from the total pressure in the globe, the 
pressure of the steam is obtained. 
Experiment I. 
0*285 grain of water introduced into globe. 
Time. 
Temperature 
in oil-bath, 
Pahr. 
Cathetometer, 
in inches. 
Column of 
mercury, in 
inches. 
Temperature 
of air, 
Fahr. 
Remarks. 
li m 
250 
4 46 
10*96 
25-57 
58 
Barometer read 30‘33 inches, or, corrected 
4 55 
250 
10*96 
25-57 
to 32° F., 30-27 in. 
5 8 
300 
11-40 
25-13 
The lower file-mark read 22-08 inches on 
the cathetometer. 
5 14 
300 
11-38 
25-15 
5 17 
300 
11-40 
25-13 
5 20 
300 
11-40 
25-13 
5 37 
250 
10-97 
25-56 
5 40 
250 
10-96 
25-57 
6 0 
200 
10-63 
25-90 
6 8, 
200 
10-60 
25-93 
6 13 
200 
10-60 
25-93 
6 45 
150 
10-28 
26-25 
6 50 
150 
10-25 
26-28 
7 25 
100 
7*90 
28-63 
Barometer read 30-36 inches, or, corrected 
7 30 
100 
7-87 
28-66 
57 
to 32° F., 30 -30 inches. 
4 M 2 
