88 THE FAIRY-LAND OF SCIENCE. 
be none, for even the delicate cobweb has been strong 
enough to shut in the heat-waves and keep the leaves 
warm. 
Again, if you walk off the grass on to the gravel 
path, you find no dew there. Why is this? Because 
the stones of the gravel can draw up heat from the 
earth below as fast as they give it out, and so they 
are never cold enough to chill the air which touches 
them. On a cloudy night also you will often find 
little or no dew even on the grass. The reason of this 
is that the clouds give back heat to the earth, and 
so the grass does not become chilled enough to draw 
the water-drops together on its surface. But after 
a hot, dry day, when the plants are thirsty and there 
is little hope of rain to refresh them, then they are 
able in the evening to draw the little drops from the 
air and drink them in before the rising sun comes 
again to carry them away. 
But our rain-drop undergoes other changes stran- 
ger than these. Till now we have been imagining it to 
travel only where the temperature is moderate enough 
for it to remain in a liquid state as water. But sup- 
pose that when it is drawn up into the air it meets with 
such a cold blast as to bring it to the freezing point. 
If it falls into this blast when it is already a drop, then 
it will freeze into a hailstone, and often on a hot sum- 
mer's day we may have a severe hailstorm, because 
the rain-drops have crossed a bitterly cold wind as 
they were falling, and have been frozen into round 
drops of ice. 
But if the water-vapour reaches the freezing air 
