THE AERIAL OCEAN IN WHICH WE LIVE. 69 
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 Ibs., and so we know that the weight of 
the air is 15 Ibs. upon every square inch,, but if the bore 
of the tube is only half a 
square inch, and there- 
fore the 30 inches of 
mercury only weigh 7^ 
Ibs. instead of 15 Ibs., the 
pressure of the atmos- 
phere 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 nar- 
row. Fig. 1 8 is a pic- 
ture of the ordinary up- 
right barometer ; the cup 
of mercury in which the 
tube stands is hidden in- 
side the round piece of 
wood A, and just at the 
bottom of this round is a small hole B, through which 
the air gets to the cup. 
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 
6 
FIG. 17. Tube of mercury in- 
verted in a basin of mercury. 
