64 THE FAIRY-LAND OF SCIENCE. 
up in the arm on which my thumb rests. Why is 
this ? Because my thumb keeps back the air from 
pressing at that end, while 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 making 
it balance a co- 
lumn of water or 
u r 'A 
Un^TyffiL In the case f the 
mJK^Jl^lii^ wetted leather we 
gUl jf&Or^ felt the weight of 
the air, here we 
A, Water in a U tube under natural pressure . ct . 
of air. 
B, Water kept in one arm of the tube by Now when WC 
pressure of the air being at the open end only \vish to SCC 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 glass filled with mercury or quicksilver, and turned 
upside-down in a small cup of mercury (see B, 
Fig. 1 6). The tube is a little more than 30 inches 
long, and though it is quite full of mercury 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 
