THE VOICES OF NATURE. \^\ 
by the air in the shell pulsating to and fro. And how 
do you think it is set going? By the throbbing of the 
veins in your own ear, which causes the air in the shell 
to vibrate. 
Another grand voice of nature is the thunder. 
People often have a vague idea that thunder is pro- 
duced by the clouds knocking together, which is very 
absurd, if you remember that clouds are but water- 
dust. The most probable explanation of thunder is 
much more beautiful than this. You will remember 
from Lecture III that heat forces the air- atoms apart. 
Now, when a flash of lightning crosses the sky it sud- 
denly expands the air all round it as it passes, so that 
globe after globe of sound-waves is formed at every 
point across which the lightning travels. Now light, 
you remember, travels with such wonderful rapidity 
(192,000 miles in a second) that a flash of lightning is 
seen by us and is over in a second, even when it is two 
or three miles long. But sound comes slowly, taking 
five seconds to travel half a mile, and so all the sound- 
waves at each point of the two or three miles fall on 
our ear one after the other, and make the rolling thun- 
der. Sometimes the roll ts made even longer by the 
echo, as the sound-waves are reflected to and fro by 
the clouds on their road; and in the mountains we 
know how the peals echo and re-echo till they die 
away. 
We might fill up far more than an hour in speak- 
ing of those voices which come to us as nature is at 
work. Think of the patter of the rain, how each drop 
as it hits the pavement sends circles of sound-waves 
out on all sides; or the loud report which falls on the 
