REPORT ON RADIANT HEAT. 
271 
attached to the face of each away from the ball, towards its 
central part; the bulb being kept in contact by the spring of a 
wire with which the thermometer was fastened. 
The results are: 1 st, That the additional diminution occasioned 
by the second screen, is proportionally much smaller than that 
occasioned by the first. Thus De la Roche’s conclusion is 
shown to hold good, not only in the case of luminous, but also 
of non-luminous hot bodies ; which is perhaps of consequence, 
as I believe doubt has been entertained respecting it; and it 
may be remarked, that here the greater thickness of the second 
screen would be against such a result, Sndly, If the progress 
of the indications of the direct effect be followed, it appears 
that the rise in the first 80 seconds is the greatest, and that those 
in the subsequent periods gradually diminish, ordly, With one 
screen, the effect in the first period is equal to, or even less 
than those in the subsequent ones ; and if we follow the tempe¬ 
rature of the first screen, it appears to sustain a rapid increase 
at first, and afterwards continues gradually to rise till some 
time after the focal thermometer has become stationary. The 
progress of the focal thermometer exactly accords with what 
must be the heating effect of the screen as a source, viz. rising 
slowly at first as the screen acquires heat sufficient to sup¬ 
ply it, and remaining stationary so long as the still increasing 
temperature of the screen could balance its loss of heat. 
4thly, With two screens, there is no rise till the second half¬ 
minute, when it is not greater than in the next half, after which 
the thermometer becomes stationary; and this trifling effect 
exactly accords with what the temperature of the second screen 
should produce. It does not begin till the second screen has 
acquired a higher temperature, and it is stationary while the 
temperature of the screen continues to increase ; and the tempe¬ 
rature 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 heating during the first part of the time, it 
follows, that a greater proportional temperature should be com¬ 
municated to the second screen by the first, than to the first by 
the iron ball. 
Other circumstances will partially cooperate in producing 
this effect,—as the greater proximity of the second screen to the 
thermometer; also more heat might be lost in communicating 
an equable temperature to the first screen from its central and 
