s3o Rev. Mr. Powell's experiments relative to 
increase ; and the temperature of the second screen is such as is 
clearly accounted for from the heating effect of the first. It 
does not begin to rise till after that of the first has risen : it 
continues stationary some time after the first has begun to 
cool, as the first screen did when the iron was cooling. But, 
as in this case, the source of heat was cooling during the 
whole time of the experiment, whilst in the other it was heat- 
ing during the first part of the time, it follows, that a greater 
proportional temperature should be communicated to the 
second screen by the first, than to the first by the iron ball. 
Other circumstances will partially co-operate in producing 
this effect ; as the greater proximity of the second screen to the 
thermometer : also more heat might be lost in communicat- 
ing an equable temperature to the first screen from its central 
and most heated part ; whilst the heat would be thus more 
equally radiated to all parts of the second without such loss. 
Thus it appears that the fact stated by M. De La Roche 
is fully substantiated ; while on the other hand it is satisfac- 
torily accounted for, without supposing any new property of 
heat, or any direct radiation through glass. 
(10.) I have been more particularly led to this enquiry, 
from becoming acquainted with some experiments on the 
subject by Mr. Ritchie, published in the Edinburgh Philoso- 
phical Journal, No. XXII. p. 281 ; in which, among other 
conclusions (to which I shall presently advert), he maintains, 
that at high temperatures below luminosity a portion of the 
heat radiates through glass of ordinary thickness, but with- 
out giving any details by which it can be made to appear 
that the effect is not sufficiently accounted for by secondary 
radiation. He has also given an explanation on his own 
