way of acting upon them. Thus, in 1767, Professor Black, of
Edinburgh, announced in his class that a vessel, filled with
hydrogen, would rise naturally in the air; but he never made the
experiment, regarding the fact as capable of being employed only
for amusement. Finally, Cavallo, in 1782, communicated to the
Royal Society of London the experiments he had made, and which
consisted in filling soap-bubbles with hydrogen. The bubbles
rose in the atmosphere, the gas which filled them being lighter
than air.



Chapter III. The Theory of Balloons.

A certain proposition in physics, known as the "Principle of
Archimedes," runs to the following effect:--"Every body plunged
into a liquid loses a portion of its weight equal to the weight
of the fluid which it displaces." Everybody has verified this
principle, and knows that objects are much lighter in water than
out of it; a body plunged into water being acted upon by two
forces--its own weight, which tends to sink it, and resistance
from below, which tends to bear it up. But this principle
applies to gas as well as to liquids--to air as well as to water.
When we weigh a body in the air, we do not find its absolute
weight, but that weight minus the weight of the air which the
body displaces. In order to know the exact weight of an object,
it would be necessary to weigh it in a vacuum.

If an object thrown into the air is heavier than the air which it
displaces, it descends, and falls upon the earth; if it is of