86 THE FAIRY-LAND OF SCIENCE. 
the air, the sunbeams come to us deprived of some of 
their heat-waves, and we can remain in the sunshine 
without suffering from the heat. 
This is how the water-vapour shields us by day, 
but by night it is still more useful. During the day 
our earth and the air near it have been storing up the 
heat which has been poured down on them, and at 
night, when the sun goes down, all this heat begins to 
escape again. Now, if there were no vapour in the air, 
this heat would rush back into space so rapidly that 
the ground would become cold and frozen even on a 
summer's night, and all but the most hardy plants 
would die. But the vapour which formed a veil against 
the sun in the day, now forms a still more powerful 
veil against the escape of the heat by night. It shuts 
in the heat-waves, and only allows them to make their 
way slowly upward from the earth thus producing 
for us the soft, balmy nights of summer and prevent- 
ing all life being destroyed in the winter. 
Perhaps you would scarcely imagine at first that it 
is this screen of vapour which determines whether or 
not we shall have dew upon the ground. Have you 
ever thought why dew forms, or what power has been 
at work scattering the sparkling drops upon the grass ? 
Picture to yourself that it has been a very hot sum- 
mer's day, and the ground and the grass have been 
well warmed, and that the sun goes down in a clear 
sky without any clouds. At once the heat-waves 
which have been stored up in the ground, bound back 
into the air, and here some are greedily absorbed by 
the vapour, while others make their way slowly up- 
ward. The grass, especially, gives out these heat-waves 
