442 
Waste. 
[August, 
their power. But the proportion of the sun’s radiations 
which fall upon any of the planets is an infinitesimal frac- 
tion of the whole. The bulk of the energy evolved is, as 
far as warming planets is concerned, simply thrown away. 
If we were to kindle the largest fire that ever blazed in the 
kitchen of a feudal castle, and to suspend before it a gnat, 
our extravagance would be a type of what is actually going 
on in the relations between sun and planet. Still this is a 
point upon which we can lay but little weight, since, for 
anything we know, the sun may have other and more im- 
portant functions than the illumination and warming of the 
planets. Therefore, in spite of the aphorism de non appa - 
rentibus et non existentibus eadem est ratio , a suspension of 
judgment is here prudent. 
Let us turn now to that portion of the sun’s rays which 
falls upon the surface of the earth. It is known that heat- 
rays as given off from the surface of incandescent bodies, 
such as metals at a white heat, or as in the case in point, 
pass through the various gases and vapours which form the 
earth’s atmosphere with little loss, so long as these are 
transparent.* They are, however, to a considerable extent 
intercepted by aqueous vapour in that condition in which it 
forms mists and clouds. Hence the earth’s crust during 
the day-time is much more heated if the sky is clear than if 
it is overcast, a facft familiar to everyone. With the heat 
radiated out from bodies at lower temperatures, such as 
from a pipe filled with boiling water, the body of a living 
animal, or the surface of the earth after exposure to the sun- 
shine, there is a decided difference. Such radiations do not 
readily traverse glass, watery vapour, even if transparent, 
and carbonic acid gas ; whilst through certain other gases, 
such as oxygen, nitrogen, and especially hydrogen, they pass 
with comparative ease. These differences explain why a 
glass house, such as a conservatory, retains the heat which 
it receives from the sun, or from steam- or hot-water-pipes. 
The rays of the sun pass easily through the glass, and warm 
the objects within. On the other hand, the low-tension 
heat-rays thrown off from the walls, floor, &c., heated by 
the sun or from the hot-water-pipes, is to a great degree 
imprisoned. Still more strikingly is this shown in the so- 
called “ solar engine.” A toiler with a blackened surface 
encased in glass is exposed to the sun’s rays, which enter 
readily and heat the metal and the water. Again, the low- 
tension rays from the surface of the boiler cannot readily 
* Transparent watery vapour absorbs about io per cent of the sun’s rays. 
