RADIATION IN ABSOLUTE MEASURE. 
599 
vacuum it was excessively tedious, several hours being required for the heating. The 
process was carried on till the thermo-junction showed that the boiling-point of water 
had been very nearly reached by the suspended globe. 
Then, preparations having previously been made, the lamp was extinguished ; the 
boiling water was very quickly run out with the help of sij)hons, and cold water run 
in, in place of it. A supply of cold water, at very nearly the temperature of the 
laboratory, was kept in readiness for this process, and a plentiful supply was run 
through the water-bath until all the surroundings had come to a steady temperature. 
Neighbouring parts of the table, retort stands, &c., were often cooled by wetting to 
obtain subsequent steadiness of temperature. In my experiments I had the advantage 
of the able assistance of Mr. A. Tanakadate. I cannot sufficiently express my, 
indebtedness to him, either for his dexterous manual aid, or for his many ingenious 
and valuable suggestions during the progress of the work. Single-handed, some of 
the work would have been impossible; and there are few, even of trained experi¬ 
menters, who could render such help as he gave me. 
As soon as the galvanometer could be read, after the cold water had been applied, 
regular observation commenced, and the method of observation was as follows :— 
It was found that an observation every 5 minutes was sufficient. (In experiments at 
higher temperatures, described below, the intervals were 2-| minutes.) A chrono¬ 
meter, beating half-seconds, stood close by; and by listening to the beats, as in 
astronomical observations, the galvanometer reading was taken at 10 seconds before the 
exact minute. The reverser was instantly shifted, and the first reading then recorded. 
By the time that 4 or 5 seconds after the exact minute was reached, the observer was 
once more ready, looking through the telescope and counting the seconds, and the 
vibrations of the galvanometer (well damped"^) had long before completely subsided ; 
and, at exactly 10 seconds after the minute, the observer was able to take the second 
reading. The sum of these readings, to right and left of the galvanometer zero, was 
taken to represent the temperature at the exact minute noted; and the mean of the 
pairs of readings, if it agreed with the known zero of the galvanometer, afforded a check, 
and showed (1) that no mistake had been made in noting ; (2) that the galvanometer 
needle was 7iot on the swing at the second reading; and (3) that there was no disturbance 
from the distant dynamo or from the moving about of magnets in the laboratory. 
The readings were at once plotted down on squared paper ; and at first this 
plotting revealed little faults hi observation, shakiness of apparatus, change of 
resistance in the leads, &c., all of which faults were soon eliminated. The curves 
then became so perfectly regular as to leave nothing to be desired. 
Before giving an account of the results obtained, I will briefly describe a modified 
* Damped by the air damping of a Thomson dead-beat galvanometer plug. It seems strange that the 
Thomson dead-beat plug is so little known to instrument makers, and to users of the reflecting galvano¬ 
meter. It has been in use in all the galvanometers sunplied by James White for about twenty-five years, 
and no instrument of the kind which is not dead-beat would be tolerated in the Glasgow laboratory. 
