equently the most buoyant of all known
gases. It is secured commercially by treating zinc or iron with dilute
sulphuric or hydrochloric acid. The average cost may be safely placed
at $10 per 1,000 feet so that, to inflate a balloon of the size of the
Zeppelin, holding 460,000 cubic feet, would cost $4,600.

Proportions of Materials Required.

In making hydrogen gas it is customary to allow 20 per cent for loss
between the generation and the introduction of the gas into the balloon.
Thus, while the formula calls for iron 28 times heavier than the weight
of the hydrogen required, and acid 49 times heavier, the real quantities
are 20 per cent greater. Hydrogen weighs about 0.09 ounce to the cubic
foot. Consequently if we need say 450,000 cubic feet of gas we must have
2,531.25 pounds in weight. To produce this, allowing for the 20 percent
loss, we must have 35 times its weight in iron, or over 44 tons. Of acid
it would take 60 times the weight of the gas, or nearly 76 tons.

In Time of Emergency.

These figures are appalling, and under ordinary conditions would be
prohibitive, but there are times when the balloon operator, unable to
obtain water or coal gas, must foot the bills. In military maneuvers,
where the field of operation is fixed, it is possible to furnish
supplies of hydrogen gas in portable cylinders, but on long trips
where sudden leakage or other cause makes descent in an unexpected spot
unavoidable, it becomes a question of making your own hydrogen gas or
deserting the balloon. And when this occurs the balloonist is up against
another serious proposition--can he find the necessary zinc or iron? Can
he get the acid?

Balloons for Commercial Use.

Despite all this the balloon has its uses. If there is to be such a
thing as aerial navigation in a commercial way--the carrying of freight
and passengers--it will come through the employment of such monster
balloons as Count Zeppelin is building. But even then the carrying
capacity must of necessity be limited. The latest Zeppelin creation,
a monster in size, is 450 feet long, and 42 1/2 feet in diameter. The
dimensions are such as to make all other balloons look like pigmies;
even many ocean-going steamers are much smaller, and yet its passenger
capacity is very small. On its 36-hour flight in May, 1909, the
Zeppelin, carried only eight passengers. The speed, however, was quite
respectable, 850 miles being covered in the 36 hours, a trifle over 23
miles an ho