68 
THE FAIRY-LAND OF SCIENCE. 
FIG. 16. A, water in a U tube under 
natural pressure of air ; B, water 
kept in one arm of the tube by 
pressure of the air being at the 
open end only at c. 
glass filled with mercury or quicksilver, and turned 
upside-down in a small cup of mercury (see B, Fig. 
17). The tube is 
a little more than 
30 inches long, 
and though it is 
quite full of mer- 
cury before it is 
turned up (A), yet 
directly it stands 
in the cup the 
mercury falls, till 
there is a height 
of about 30 inches between the surface of the mercury 
in the cup C, and that of the mercury in the tube B. 
As it falls it leaves an empty space above the mercury 
at B which is called a vacuum, because it has no air in 
it. Now, the mercury is under the same conditions as 
the water was in the U tube, there is no pressure upon 
it at B, while there is a pressure of 15 Ibs. upon it in the 
bowl, and therefore it remains held up in the tube. 
But why will it not remain more than 30 inches 
high in the tube? You must remember it is only 
kept up in the tube at all by the air which presses on 
the mercury in the cup. And that column of mercury 
C B now balances the pressure of the air outside, and 
presses down on the mercury in the cup at its mouth 
just as much as the air does on the rest. So this cup 
and tube act exactly like a pair of scales. The air out- 
side is a thing to be weighed at one end as it presses 
on the mercury, the column C B answers to the leaden 
weight at the other end which tells you how heavy 
