64 THE FAIRY-LAND OF SCIENCE 
up in the arm on which my thumb rests. Why ii 
this ? Because my thumb keeps back the air frorr 
pressing at that end, while the whole weight of the 
atmosphere rests on the water at c. And so w< 
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 01 
any other liquid 
In the case of th< 
wetted leather w< 
felt the weight o 
the air, here w< 
A, Water in a U tube under natural pressure -^ e ff ec ^ s 
of air. 
B, Water kept in one arm of the tube by WOW when W( 
pressure of the air being at the open end only wish to SCC th< 
at '- weight of the ai 
we consult a barometer, which works really just ir 
the same way as the water in this tube. An ordi 
nary upright barometer is simply a straight tub< 
of glass filled with mercury or quicksilver, and turnec 
upside-down in a small cup of mercury (see B 
Fig. 1 6). The tube is a little more than .30 inche 
long, and though it is quite full of mercury before i 
is turned up (A), yet directly it stands in the cup thi 
mercury falls, till there is a height of about 30 inche 
between the surface of the mercury in the cup C, an< 
that of the mercuiy in the tube B. As it falls it leave 
an empty space above the mercury at B which is calle( 
a vacuum, because it has no air in it. Now, the mercun 
is under the same conditions as the water was in th 
U tube, there is no pressure upon it at B, while ther 
