66 
THE FAIRY-LAND OF SCIENCE. 
will also be halved, because it will only act upor 
half a square inch of surface, and for this reason il 
, Fig. 17. will make no difference to the heighl 
of the mercury whether the tube be 
broad or narrow. Fig. 17 is a picture 
of the ordinary upright barometer ; the 
cup of mercury in which the tube stands 
is hidden inside the round piece ol 
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 wil 
show this at once, because there will 
be less weight on the mercury in the 
cup, therefore it will not keep the mer- 
cury pushed so high up in the tube, 
In other words, the mercury in the 
tube will fall. 
Let us suppose that one day the aii 
is so much lighter that it presses down 
only with a weight of 14^ Ibs. to the 
square inch instead of 15 Ibs. Then 
the mercury would fall to 29 inches, 
because each inch is equal to the 
Wei S ht f half a P OUnd N , when 
A, Wood covering the air is damp and very full of water- 
cup of mercury. . . 11-1' 
B, Hole through vapour it is much lighter, and so when 
which air acts, the barometer falls we expect rain, 
Sometimes, however, other causes make the air light. 
and then, although the barometer is low, no rain comes, 
