A DROP OF WATER. 83 
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 upwards from the earth thus 
producing for us the soft, balmy nights of summer 
and preventing 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 summer'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- 
wards. The grass, especially, gives out these heat- 
waves very quickly, because the blades, being very 
G 2 
