THE AERIAL OCEAN IN WHICH WE LIVE. 67 
tumbler, kept there entirely by the air pressing upward 
against it. 
And now we are almost prepared to understand 
how we can weigh the invisible air. One more experi- 
ment first. I have here (Fig. 16, p. 68) what is called 
a U tube, because 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. 16), 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. 16). But the water does not 
now return to its even position, it remains up in 
the arm on which my thumb 
rests. Why is this ? Because 
my thumb keeps back the 
air from pressing at that end, 
and the whole weight of the 
atmosphere rests on the water 
at c. And so we learn that 
not only has the atmosphere 
real weight, but we can see the 
effects of this weight by mak- 
ing it balance a column of 
water or any other liquid. In 
the case of the wetted leather we felt the weight of the 
air, here we see its effects. 
Now when we wish to see the weight of the air 
we consult a barometer, which works really just in 
the same way as the water in this tube. An ordi- 
nary upright barometer is simply a straight tube of 
