62 THE FAIRY-LAND OP SCIENCE. 
pop-gun; they are obliged to submit to be pressed 
together. 
Even a short distance from the earth, however, at 
the top of a high mountain, the air becomes lighter, 
because it has less weight of atmosphere above it, and 
people who go up in balloons often have great diffi- 
culty in breathing, because the air is so thin and light. 
In 1804 a Frenchman, named Gay-Lussac, went up 
four miles and a half in a balloon, and brought down 
some air; and he found that it was much less heavy 
than the same quantity of air taken close down to the 
earth, showing that it was much thinner, or rarer, as it 
is called; * and when, in 1862, Mr. Glaisher and Mr. 
Coxwell went up five miles and a half, Mr. Glaisher's 
veins began to swell, his head grew dizzy, and he 
fainted. The air was too thin for him to breathe 
enough in at a time, and it did not press heavily 
enough on the drums of his ears and the veins of his 
body. He would have died if Mr. Coxwell had not 
quickly let off some of the gas in the balloon, so that 
it sank down into denser air. 
And now comes another very interesting question. 
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 
* 100 cubic inches near the earth weighed 31 grains, while the 
same quantity taken at four and a half miles up in the air 
weighed only 12 grains, or two-fifths of the weight. 
