THE AERIAL OCEAN IN WHICH WE LIVE. 59 
And now comes another very interesting questioa 
If the air gets less and less dense as it is farther from 
the earth, where does it stop altogether ? We cannot 
go up to find out, because we should die long before 
we reached the limit ; and for a long time we had to 
guess about how high the atmosphere probably was, 
and it was generally supposed not to be more than fifty 
miles. But lately, some curious bodies, which we 
should have never suspected would be useful to us in 
this way, have let us into the secret of the height of 
the atmosphere. These bodies are the meteors, or 
falling stars. 
Most people, at one time or another, have seen what 
looks like a star shoot right across the sky, and dis- 
appear. On a clear starlight night you may often see 
one or more of these bright lights flash through the 
air ; for one falls on an average in every twenty 
minutes, and on the nights of August 9th and No- 
vember 1 3th there are numbers in one part of the 
sky. These bodies are not really stars ; they are 
simply stones or lumps of metal flying through the 
air, and taking fire by clashing against the atoms of 
oxygen in it. There are great numbers of these 
masses moving round and round the sun, and when 
our earth comes across their path, as it does espe- 
cially in August and November, they dash with such 
tremendous force through the atmosphere that they 
grow white-hot, and give out light, and then dis- 
appear, melted into vapour. Every now and then 
one falls to the earth before it is all melted away, 
and thus we learn that these stones contain tin, iron, 
sulphur, phosphorus, and other substances. 
