46 THE FAIRY-LAND OF SCIENCE. 
So far we have spoken only of light ; but hold your 
hand in the sun and feel the heat of the sunbeams, and 
then consider if the waves of heat do not do work 
also. There are many waves in a sunbeam which 
move too slowly to make us see light when they hit 
our eye, but we can feel them as heat, though we 
cannot see them as light. The simplest way of feeling 
heat-waves is to hold a warm iron near your face. 
You know that no light conies from it, yet you can feel 
the heat-waves beating violently against your face and 
scorching it. Now there are many of these dark heat- 
rays in a sunbeam, and it is they which do most of 
the work in the world. 
In the first place, as they come quivering to the 
earth, it is they which shake the water 7 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. 
