the passage of radiant heat through glass screens . 379 
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-lumi- 
nous hot bodies ; which is perhaps of consequence, as I believe 
doubts have been entertained respecting it ; and it may be 
remarked, that here the greater thickness of the second screen 
would be against such a result. 
2dly. If the progress of the indications of the direct effect 
be followed, it appears that the rise in the first 30 seconds is 
the greatest, and those in the subsequent periods gradually 
diminish. 
sdly. 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 temperature of the first screen, it appears to sustain 
a rapid increase at first, and afterwards continues gradually 
to rise till sometime 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 supply 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 2d 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 
