46 THE FAIRY-LAND OF SCIENCE. 
In the first place, as they come quivering to the 
earth, it is they which shake the water-drops apart, so 
that these are carried up in the air, as we shall see in 
the next lecture. And then remember, it is these 
drops, falling again as rain, which make the rivers and 
all the moving water on the earth. So also it is the 
heat-waves which make the air hot and light, and 
so cause it to rise and make winds and air-currents, 
and these again give rise to ocean-currents. It is 
these dark rays, again, which strike upon the land and 
give it the warmth which enables plants to grow. It 
is they also which keep up the warmth in our own 
bodies, both by coming to us directly from the sun, 
and also in a very roundabout way through plants. 
You will remember that plants use up rays of light 
and heat in growing ; then either we eat the plants, or 
animals eat the plants and we eat the animals; and 
when we digest the food, that heat comes back in our 
bodies, which the plants first took from the sunbeam. 
Breathe upon your hand, and feel how hot your breath 
is ; well, that heat which you feel, was once in a sun- 
beam, and has travelled from it through the food you 
have eaten, and has now been at work keeping up the 
heat of your body. 
But there is still another way in which these plants 
may give out the heat-waves they have imprisoned. 
You will remember how we learnt in the first lecture 
that coal is made of plants, and that the heat they 
give out is the heat these plants once took in. Think 
how much work is done by burning coals. Not only 
are our houses warmed by coal fires and lighted by 
coal gas, but our steam-engines and machinery work 
