each of the globes would be about 1800 tons. Something more than a
thin copper shell would be needed to resist this crushing force and
an adequate increase in the strength of the shells would so enhance
their weight as to destroy their lifting power.

[Illustration: Lana's Vacuum Balloon.]

To tell at length the stories of attempt and failure of the earliest
dabblers in aeronautics would be unprofitable and uninteresting. Not
until the eighteenth century did the experimenters with
lighter-than-air devices show any practical results. Not until the
twentieth century did the advocates of the heavier-than-air machines
show the value of their fundamental idea. The former had to discover
a gaseous substance actually lighter, and much lighter, than the
surrounding atmosphere before they could make headway. The latter
were compelled to abandon wholly the effort to imitate the flapping
of a bird's wings, and study rather the method by which the bird
adjusts the surface of its wings to the wind and soars without
apparent effort, before they could show the world any promising
results.

Nearly every step forward in applied science is accomplished because
of the observation by some thoughtful mind of some common phenomenon
of nature, and the later application of those observations to some
useful purpose.

It seems a far cry from an ancient Greek philosopher reposing
peacefully in his bath to a modern Zeppelin, but the connection is
direct. Every schoolboy knows the story of the sudden dash of
Archimedes, stark and dripping from his tub, with the triumphant cry
of "Eureka!"--"I have found it!" What he had found was the rule
which governed the partial flotation of his body in water. Most of
us observe it, but the philosophical mind alone inquired "Why?"
Archimedes' answer was this rule which has become a fundamental of
physics: "A body plunged into a fluid is subjected by this fluid to
a pressure from below to above equal to the weight of the fluid
displaced by the body." A balloon is plunged in the air--a fluid. If
it is filled with air there is no upward pressure from below, but if
it is filled with a gas lighter than air there is a pressure upward
equal to the difference between the weight of that gas and that of
an equal quantity of air. Upon that fact rests the whole theory and
practice of ballooning.

The illustration of James Watt watching the steam rattle the cover
of a teapot and from it getting the rudimentary idea o