SUNBEAMS AND THEIR WORK. 33 
frozen to death if the sun were cold, so we should 
all be burnt up with intolerable heat if his fierce rays 
fell with all their might upon us. But we have an 
invisible veil protecting us, made of what do you 
think ? Of those *tiny particles of water which the 
sunbeams draw up and scatter in the air, and which, 
as we shall see in Lecture IV., cut off part of the in- 
tense heat and make the air cool and pleasant for us. 
We have now learnt something of the distance, the 
size, the light, and the heat of the sun the great 
source of the sunbeams. But we are as yet no nearer 
the answer to the question, What is a sunbeam ? how 
does the sun touch our earth ? 
Now suppose I wish to touch you from this plat- 
form where I stand, I can do it in two ways. Firstly, 
I can throw something at you and hit you in this 
case a thing will have passed across the space from 
me to you. Or, secondly, if I could make a violent 
movement so as to shake the floor of the room, you 
would feel a quivering motion ; and so I should touch 
you across the whole distance of the room. But in 
this case no tiling would have passed from me to you 
but a movement or wave, which passed along the 
boards of the floor. Again, if I speak to you, how 
does the sound reach your ear? Not by anything 
being thrown from my mouth to your ear, but by 
the motion of the air. When I speak I agitate the 
air near my mouth, and that makes a wave in the 
air beyond, and that one, another, and another (as 
we shall see more fully in Lecture VI.), till the last 
wave hits the drum of your ear. 
D 
