rate propelling machinery. Though the suggestion was not
developed to any useful point it was of interest as forecasting the
fundamental idea of the gas engines of to-day which have made
aviation possible--that is, the creation of power by a series of
explosions within the motor.

In the effort to make balloons dirigible one of the first steps was
to change the form from the spherical or pear-shaped bag to a
cylindrical, or cigar-shape. This device was adopted by the brothers
Robert in France as early as 1784. Their balloon further had a
double skin or envelope, its purpose being partly to save the gas
which percolated through the inner skin, partly to maintain the
rigidity of the structure. As gas escapes from an ordinary balloon
it becomes flabby, and can be driven through the air only with
extreme difficulty. In the balloon of the Robert brothers air could
from time to time be pumped into the space between the two skins,
keeping the outer envelope always fully distended and rigid. In
later years this idea has been modified by incorporating in the
envelope one large or a number of smaller balloons or "balloonets,"
into which air may be pumped as needed.

The shape too has come to approximate that of a fish rather than a
bird, in the case of balloons at least. "The head of a cod and the
tail of a mackerel," was the way Marey-Monge, the French aeronaut
described it. Though most apparent in dirigible balloons, this will
be seen to be the favourite design for airplanes if the wings be
stripped off, and the body and tail alone considered. Complete,
these machines are not unlike a flying fish.

In England, Sir George Cayley, as early as 1810 studied and wrote
largely on the subject of dirigibles but, though the English call
him the "father of British aeronautics," his work seems to have been
rather theoretical than practical. He did indeed demonstrate
mathematically that no lifting power existed that would support the
cumbrous steam-engine of that date, and tried to solve this dilemma
by devising a gas engine, and an explosive engine. With one of the
latter, driven by a series of explosions of gunpowder, each in a
separate cell set off by a detonator, he equipped a flying machine
which attained a sufficient height to frighten Cayley's coachman,
whom he had persuaded to act as pilot. The rather unwilling aviator,
fearing a loftier flight, jumped out and broke his leg. Though by
virtue of this martyrdom his name should su