tube, 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.
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 the
other end. 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 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 ordinary upright barometer is simply a straight
tube of glass filled with mercury or quicksilver, and turned
upside-down in a small cup of mercury. The tube is a little more
than 30 inches long, and though it is quite full of mercury
before it is turned up, 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, and that of the mercury in
the tube. As it falls it leaves an empty space above the mercury
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 the top of the tube, while
there is a pressure of 15 lbs. upon it in the bowl, and therefore
it remains held up in the tube.