THE AERIAL OCEAN IN WHICH WE LIVE. 63 
understand how, being elastic, it also presses upwards ; 
and we can prove this by a simple experiment. I 
fill this tumbler with water, and keeping a piece of 
card firmly pressed against it, I turn the whole upside- 
down. When I now 
take my hand away 
you would naturally 
expect the card to 
fall, and the water to 
be spilt. But no ! the 
card remains as if 
glued to the tumbler, 
kept there entirely 
by the air pressing 
upwards against it.* Averted tumbler of water with card kept 
against it by atmospheric pressure. 
And now we are 
almost prepared to understand how we can weigh the 
invisible air. One more experiment first. I have 
here (Fig. 15, p. 64) what is called a U tube, be- 
cause it is shaped like a large U. I pour some 
water in it till it is about half full, and you will 
notice that the water stands at the same height 
in both arms of the tube (A, Fig. 15), because the 
air presses on both surfaces alike. Putting my 
thumb on one end I tilt the tube carefully, so as 
to make the water run up to the end of one arm, and 
then turn it back again (B, Fig. 15). But the water 
does not now return to its even position, it remains 
* The engraver has drawn the tumbler only half full of water. Thie 
experiment will succeed quite as well in this way if the tumbler be 
turned over quickly, so that part of the air escapes between the tumbler 
and the card, and therefore the space above the water is occupied by 
air less dense than that outside. 
