In classifying air craft, the sailing balloon was mentioned as a type
intermediate between the drifting balloon and the dirigible. No such
type has before been recognized: but it may prove to have its field,
just as the sailing vessel on the sea has bridged the gap between the
raft and the steamship. It is true that tacking is impossible, so that
our sailing balloons must always run before the wind: but they possess
this great advantage over marine sailing craft, that by varying their
altitude they may always be able to find a favorable wind. This implies
adequate altitude control, which is one of the problems not yet solved
for lighter-than-air flying machines: but when it has been solved we
shall go far toward attaining a dirigible balloon without motor or
propeller; a true sailing craft.
[Illustration: DIAGRAM OF PARTS OF A DRIFTING BALLOON]
This means more study and careful utilization of stratified atmospheric
currents. Professor Rotch suggests the utilization of the upper
westerly wind drift across the American continent and the Atlantic
Ocean, which would carry a balloon from San Francisco to southern
Europe at a speed of about fifty feet per second--thirty-four miles
per hour. Then by transporting the balloon to northern Africa, the
northeast surface trade wind would drive it back to the West Indies at
twenty-five miles per hour. This without any motive power: and since
present day dirigibles are all short of motive power for complete
dirigibility, we must either make them much more powerful or else adopt
the sailing principle, which will permit of actually decreasing present
sizes of motors, or even possibly of omitting them altogether. Our next
study is, then, logically, one of altitude control in balloons.
[Illustration: GLIDDEN AND STEVENS GETTING AWAY IN THE "BOSTON"
(Leo Stevens, N.Y.)]