372 
MR. E. H. GRIFFITHS OH THE VALUE OF 
air-thermometer modify our determinations of differences of temperature (and it is 
only differences which are important to us), our results will be modified accordingly. 
This will not, however, necessitate any repetition of the experimental work, as the 
corrections involved will be of a numerical order only. 
Measurement of the Heat generated hy Stirring. 
The difficulties that we have met with in this portion of our work have been very 
great. As will be shown in Section XI., we have, against our will, been compelled to 
increase the work done by the stirrer until the heat thus generated became about 1/10 
of the whole supply. We found continuous and very rapid stirring to be a necessity. 
Variations in R and in the thermometer readings and in the water equivalent of the 
calorimeter invariably followed if the mixing was insufficient. Commencing in 1888 
with a stirrer revolving at the rate of about 100 revolutions per minute, the rate was 
gradually increased until, in our last series, we aimed (with, it is true, a different 
form of stirrer) at a rate of nearly 2000 revolutions per minute. 
The accurate measurement of the heat thus generated is only second in importance 
to the determination of the heat due to the current, and, accordingly, a great portion 
of our time and attention was directed to it. Fortunately, we were able to com¬ 
pletely establish the law that, with our form of stirrer, the work done varied directly 
as r®, r being the rate of rotation. From this time our difficulties rapidly diminished. 
Radiation, Conduction, Convection, &c. 
We endeavoured, as far as possible, to diminish loss or gain due to the above 
causes. 
Bottomley* has shown that there is a rapid fall in the rate of loss or gain by radia¬ 
tion and convection when the pressure of the surrounding gas falls below 1 millim. 
Most of our observations were taken with the surrounding pressure under *5 millim. 
The calorimeter itself had a bright polished surface, and the walls of the enclosing 
chamber were clothed with bright speculum metal. In Section XI., we show that 
throughout our range of temperature, Newton’s law of radiation holds— i.e., the loss 
or gain varies directly as the differences of temperature. It must, however, be 
remembered that we have not endeavoured to separate the partial effects of the 
radiation, conduction, and convection, and hence we can only say that the combined 
effect obeys this law. The total loss or gain, due to the above causes, was 
determined separately for each change in the contained mass. 
Any error in this part of our work would have but a. small effect on the mean 
value of J, since, in each experiment (if taken as a whole), the total loss or gain by 
conduction, convection, and radiation, must have been smah. 
* ‘ Phil. Trans.,’ 1887, A, p. 446. 
