132 REPORT —1890. 
hundredth of an ampére. To illustrate the closeness of the agreement 
in the results obtained at different times I give the following determi- 
nations of the resistance at a temperature of 100° determined in the 
usual manner by means of a hypsometer with manometer attached. 
Full corrections were made in the barometric reading, and the results 
reduced to lat. 45°. 
| Resistance (after corr. for 
Date : Temperature temp. of coils) 
July 26 : : : | 100°C. 18-2029 
Fa 27 : ? a 100°C. 18-2034 
August 12 ; : : 100°C. 18-2025 
ayers} s : f 100°C. 18-2031 
Mean ; : ‘ : F s ‘ : E - 18-2030 
The expression for the platinum temperature by this thermometer was 
R—13°5219 o> 
TT 6elion CF 100, again Rear 3462, 
almost exactly agreeing with the coefficient of the wire in Mr. Callendar’s 
air thermometer (‘ Phil. Trans.’ A. 1887). 
Mr. G. M. Clark, B.A. (Sidney Coll., Cambridge), now joined me in 
the investigation, and as we proposed to use this thermometer for the 
calibration and graduation of mercury thermometers between 0° and 100°, 
we decided to obtain intermediate temperatures by means of Regnault’s 
numbers connecting the temperature and pressure of aqueous vapour. 
For this purpose we constructed a large iron tank with two plate-glass 
sides, holding about 16 gallons of water, and through two holes bored in 
the bottom inserted two barometer tubes, the upper 16 inches of each 
being within the tank. One of these was used as a standard barometer, 
and was prepared with great care, the distilled mercury with which it 
was filled having been boiled in the tube for more than six hours. The 
internal diameter of the tube was 14 mm., and the absence of any menis- 
eus was very marked. If the level of the surface of the water in the 
tank was below the top of the barometer, and the water warmed, the 
sublimation of mercury in the vacuous space was observable. The second 
barometer was made from the same length of tubing as the first, and 
communicated at its upper extremity with a small flask (A), in which was 
placed the platinum thermometer. 
Distilled water was boiled in vacuo for some hours, to expel all traces 
of air. The flask and barometer tube were then exhausted by means of 
an air-pump, and the lower end of the tube placed in a flask (B) contain- 
ing the previously boiled water, which rushed up, filling the tube and 
flask (A). : 
The water remaining in B was then boiled until this flask and a 
bent tube passing from it into a basin of mercury, 30 inches beneath, 
were completely filled with steam, and, on cooling, the height of mercury 
in the tube enabled us to determine that the pressure on its surface.was 
that of aqueous vapour only. The water in the upper flask was then 
boiled for many hours, and only allowed to cool occasionally to permit of 
the water in the lower flask being boiled away. To prevent access of air 
