44 PR A GMEKTS 0V SCIENCE. 
and becomes thereby heated, thus wrapping the earth like 
a warm garment, and protecting its surface from the deadly 
chill which it would otherwise sustain. Various philoso- 
phers have speculated on the influence of an atmospheric 
envelope. De Saussure, Fourier, M. Pouillet, and Mr. 
Hopkins, have, one and all, enriched scientific literature 
with contributions on this subject, but the considerations 
which these eminent men have applied to atmospheric air, 
have, if my experiments be correct, to be transferred to the 
aqueous vapor. 
The observations of meteorologists furnish important, 
though hitherto unconscious evidence of the influence of 
this agent. Wherever the air is dry we are liable to daily 
extremes of temperature. By day, in such places, the sun's 
heat reaches the earth unimpeded, and renders the maxi- 
mum high; by night, on the other hand, the earth's heat 
escapes unhindered into space, and renders the minimum 
low. Hence the difference between the maximum and 
minimum is greatest where the air is driest. In the plains 
of India, on the heights of the Himalaya, in central Asia, 
in Australia wherever drought reigns, we have the heat of 
day forcibly contrasted with the chill of night. In the 
Sahara itself, when the sun's rays cease to impinge on the 
burning soil, the temperature runs rapidly down to freez- 
ing, because there is no vapor overhead to check the calo- 
rific drain. And here another instance might be added to 
the numbers already known, in which nature tends, as it 
were, to check her own excess. By nocturnal refrigeration, 
the aqueous vapor of the air is condensed to water on the 
surface of the earth; and, as only the superficial portions 
radiate, the act of condensation makes water the radiating 
body. Now experiment proves that to the rays emitted by 
water, aqueous vapor is especially opaque. Hence the very 
act of condensation, consequent on terrestrial cooling, 
becomes a safeguard to the earth, imparting to its radia- 
tion that particular character which renders it most liable 
to be prevented from escaping into space. 
It might, however, be urged that, inasmuch as we derive 
all our heat from the sun, the selfsame covering which 
protects the earth from chill must also shut out the solar 
radiation. This is partially true, but only partially; the 
sun s rays are different in quality from the earth's rays, 
and it does not at all follow that the substance which 
