REPORT ON RADIANT HEAT. 
259 
Report on the present State of our Knowledge of the Science 
of Radiant Heat . By the Rev. Baden Powell, M.A. 
F.R.8., Savilian Professor of Geometry in the University of 
Oxford. 
In attempting to give a condensed account of the present state 
of our knowledge of the science of Radiant Heat , it appears to 
me that I shall be best consulting the design of such a Report 
by offering, in as brief a form as possible, a sketch of what has 
been formerly done in this department; and thence proceeding 
to a more detailed survey of what is now doing. And we shall 
proceed with greater clearness if we distinguish the several dif¬ 
ferent departments into which the subject divides itself, agree¬ 
ably to certain known distinctions in the properties and species of 
heat acting under peculiar circumstances. All these have been 
too commonly confounded together under the general and 
vague name of Radiant Heat, whence not unfrequently the most 
erroneous views have resulted. By distributing our subject, 
however, under the few well-marked divisions which the scanty 
results of observation as yet supply, we shall at once secure 
perspicuity in our views, and be treating the subject in a way 
most accordant with the inductive process; which, it must be 
distinctly avowed, has not yet enabled us to advance to any 
such comprehensive knowledge of the facts as can warrant us 
in generalizing them, or in ascribing to a common principle the 
radiation of heat from a mass of hot water, from a flame, and 
from the sun. 
We shall take each of these principal divisions separately, 
and under each shall consider what is known in reference to 
those properties to which experiment has been directed. 
Division I. 
Radiation of heat from hot bodies below the temperature 
of luminosity. 
a.) Radiation (or communication of heat to sensible di¬ 
stances,) is distinct from its conveyance by conduc¬ 
tion through the air ; since , 
1. ) It takes place perpendicularly downwards: 
2. ) Only in elastic media. 
The relative cooling in different media is seen in the foR 
lowing experiments. (Rumford’s Essays, ii. 425; Torricelli; 
Murray’s Ghent . i. 328.) 
r 2 
