58 THE FAIRY-LAND OF SCIENCE. 
last they rebel so strongly against being more crowded 
that the cork cannot resist their pressure. Out it 
flies, and the atoms spread themselves out comfortably 
again in the air all around them. Now, just as I 
pressed the air together in the pop-gun, so the atmo- 
sphere high up above the earth presses on the air 
below and keeps the atoms closely packed together. 
And in this case the atoms cannot force back the air 
above them as they did the cork in the 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, and 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. 
ioo 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 
