THE FAIRY-LAND OF SCIENCE. 
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^ Ibs. to the 
square inch instead of 15 Ibs. Then 
the mercury would fall to 29 inches, 
because each inch is equal to the 
weight of half a pound. Now, when 
the air is damp and very full of water- 
vapour it is much lighter, and so when 
the barometer falls we expect rain. 
Sometimes, however, other causes make 
the air light, and then, although the 
barometer is low, no rain comes. 
Again, if the air becomes heavier 
the mercury is pushed up above 30 to 
31 inches, and in this way we are able 
to weigh the invisible air-ocean all over 
the world, and tell when it grows lighter 
or heavier. This, then, is the secret of 
the barometer. We cannot speak of the 
thermometer to-day, but I should like 
to warn you in passing that it has noth- 
ing to do with the weight of the air, 
FIG. 18. Ordi- but only with heat, and acts in quite a 
nary upright ,.~. 
barometer different way. 
And now we have been so long 
A, wood cov- 
ering cup of 
mercury ; B, 
hole through hunting out, testing and weighing our 
which air acts. aer j a j ocean, that scarcely any time is 
left us to speak of its movements or the pleasant 
