20 
Ordinary Meeting, November 16th, 1875. 
Rev. William Gaskell, M.A., Vice-President, in the Chair. 
“ On an Instrument for Measuring the Direct Heat of the 
Sun,” by Professor Balfour Stewart, LL.D,, F.R.S. 
The instrument generally employed for giving the radiant 
energy of the sun’s rays acts upon the following principle. 
In the first place the instrument is sheltered from the sun but 
exposed to the clear sky, say for five minutes. Let the heat 
so lost be termed r. . Secondly, the instrument is turned to 
the sun for five minutes. Let the heat so gained be termed 
R. Thirdly, the instrument being now hotter than it was 
in the first operation is turned once more so as to be ex- 
posed to the clear sky for five minutes while it is shielded 
from the sun. Let the heat so lost be termed r. 
It thus appears that r denotes the heat lost by convection 
and radiation united when the instrument, before being 
heated by the sun, is exposed for five minutes to the clear 
sky, while r' denotes the heat lost by these same two opera- 
tions by a similar exposure after the instrument has been 
heated by the sun ; and it is assumed that the heat lost 
from these two causes during the time when the instru- 
ment is being heated by the sun will be a mean between 
r and r\ and hence that the whole effect of the sun’s rays 
will be in reality R F • 
Now although this assumption may in the average ol a 
great number of experiments represent the truth, yet in 
many individual cases, it may be far from being true. It 
would therefore seem to be desirable to get rid of this 
uncertainty by constructing an instrument in which we are 
sure that the causes of variability are not allowed to 
operate. 
