THE AERIAL OCEAN IN WHICH WE LIVE. 65 
Fig. 16. 
is a pressure of 15 Ibs. upon it in the bowl, and there- 
fore it remains held up in the tube. 
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 mer- 
cury C B 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 C B 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 1 5 Ibs., and so we know that the weight ot 
the air is 15 Ibs. 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^ Ibs 
instead of 15 Ibs., the pressure of the atmosphere 
F 
Tube of mercury inverted in a 
basin of mercury. 
