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. upon every square
inch, but if the bore of the tube is only half a square inch, and
therefore the 30 inches of mercury only weigh 7 1/2 lbs. instead
of 15 lbs., the pressure of the atmosphere will also be halved,
because it will only act upon half a square inch of surface, and
for this reason it will make no difference to the height of the
mercury whether the tube be broad or narrow.

But now suppose the atmosphere grows lighter, as it does when it
has much damp in it. The barometer will show this at once,
because there will be less weight on the mercury in the cup,
therefore it will not keep the mercury pushed so high up in the
tube. In other words, the mercury in the tube will fall.

Let us suppose that one day the air is so much lighter that it
presses down only with a weight of 14 1/2 lbs. to the square inch