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.
Week 9
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 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 outside is the thing to be weighed at
one end as it presses on the mercury, the column answers to the
leaden weight at the other end which tells you how heavy the air
is. Now if the bore of this tube is made an inch square, then
the 30 inches of mercury in it weigh exactly 15 lbs, and so we
know that the weight of the air is 15 lbs.
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