ON THE MECHANICAL EQUIVALENT OF HEAT. 
347 
atmosphere, so that the air does not enter the vapour chamber in which the thermo¬ 
meters are, but directly passes out with the overflow steam. 
There is necessarily some resistance to the air passing along the pipe to the vapour 
chamber, but this could easily be tested by removing the pipe from the vapour 
chamber, and leaving it open to the atmosphere, so that the barometer would adjust 
itself to that of the atmosphere, plus the pressure due to the resistance of the current 
in the pipe ; then, stopping the current by closing the branch pipe, and reading 
ao’ain, the difference would give the pressure due to the current. With the plug 
as described this was so small as to be negligible, even when the pressure in the 
receiver was two atmospheres. As during the testing of the thermometers the 
pressure in the vapour chamber was generally greater than that of the atmosphere, 
in order to maintain this steady, a governor on the gas burner was necessary, as well 
as an accurately adjustable exit valve. 
With these appliances the scale of high temperature thermometer could be tested 
at intervals, over a sufficient interval on each side of the boiling point (212° Fahr.), 
the corrections for surface tension, temperature, and gravitation being applied to within 
the thousandth of an inch of mercury. 
This sives the limits of error i 0'00001. 
Correction of the Low Temperature Thermometer. 
49. The correction on the thermometer for 32° would be at any time obtained in 
the usual way by immersing the thermometer vertically in a bath of soft snow, but 
as there w^as no ready means, as with the scale about 212°, of testing the scale at 32°, 
while this would be used for one or two degrees, this correction could only be made 
by comparison with a thermometer already corrected v/ith the air thermometer, which 
comparison Dr. Schuster allow-ed to be made in the physical department. 
Corrections of the Thermometers for Pressure. 
50. The pressures in the thermometer chambers of the brake being both some 
10 or 15 inches of iiiercury above that of the atmosphere, it would be necessary to 
determine the corrections on each of the thermometers under the pressures and 
temperatures at which they had to wmrk. 
Thus, if Cl, Cjj are the corrections per unit of pressure in the initial and final 
thermometers, the correction for the heat is {cyPi — c^p^. 
The Range of Temperature over which the Specific Heat ivould he Measured. 
51. The temperature of the effluent water from the brake can be regulated either 
up or down to any required extent, and although there would necessarily be some 
2 Y 2 
