Waste . 
1883.] 
443 
escape, and the water is thus brought to a boil. In cloud- 
less regions a steam-engine may thus be made to work. 
The same faCts play an important part in Nature. During 
the day, as we have already seen, the earth is gaining heat, 
if only the sky is clear. During the night there is a loss 
going on, the heat which has been collected during the day 
being dissipated by radiation into space. This loss, which 
is always experienced to some extent, is greatest when the 
sky is cloudless by night, as is always observed in case of 
the spring-frosts so much dreaded by the farmer and the 
gardener. Hence, for the earth to receive the greatest pos- 
sible effect of the sun’s rays, the sky should be clear by day 
and overcast by night. But, in England at least, we might 
almost say that the very reverse order prevails. The sun is 
visible but for a part of the time when he is above the hori- 
zon,-— in some seasons not more than one-fourth,— -whilst 
the heavens clear up at night. Were accurate observations 
made we suspeCt that the sky would be found cloudless by 
night for twice as many hours yearly as in the day. Here, 
therefore, we have a double loss : a large part of the sun’s 
rays are shut off from us by day, whilst by night the heat 
which the earth possesses is allowed to escape freely. Here, 
then, is twofold waste on a gigantic scale. So fully has the 
evil of clear nights been recognised that in many parts of 
the world it has been customary to protect orchards and 
vineyards from night-frosts in spring by making a dense 
smoke to the windward. 
Transparent watery vapour is also a great protection 
against the nightly loss of heat. Where it is wanting, no 
intensity of solar aCtion by day is able to ward off frost, 
even within the tropics. In the deserts of Africa, Arabia, 
and Persia travellers often experience bitter cold before sun- 
rise. The so-called “ extreme ” climates, where a scorching 
summer alternates with a severe winter, have a compara- 
tively dry atmosphere. The chills of the easterly winds of 
March and April, which few of us “ welcome,” as did 
Kingsley, owe their evil effects upon man, beast, and vege- 
tation to the same lack of transparent watery vapour. 
We have already mentioned the power of imprisoning 
low-tension heat-rays possessed by carbonic acid, as com- 
pared with oxygen and nitrogen. So manifest is this pro- 
perty that some writers have sought to explain the luxuriant 
vegetation of the Eocene and Miocene Ages, even in high 
latitudes, by the assumption of a much larger proportion of 
carbonic acid in the atmosphere than exists in our days. 
In all probability it may be said that if nitrogen possessed 
2 G 2 
