Page:Tom Swift and His Air Glider.djvu/77

Rh day, and he had an additional helper, for Mr. Petrofsky proved to be a good mechanic.

In brief, the air glider was like an aeroplane save that it had no motor. It was raised by a strong wind blowing against transverse planes, and once aloft was held there by the force of the air currents, just like a box kite is kept up. To make it progress either with or against the wind, there were horizontal and vertical rudders, and sliding weights, by which the equilibrium could be shifted so as to raise or lower it. While it could not exactly move directly against the wind it could progress in a direction contrary to which the gale was blowing, somewhat as a sailing ship "tacks."

And, as has been explained, the harder the wind blew the better the air glider worked. In fact unless there was a strong gale it would not go up.

"But it will be just what is needed out there in that part of Siberia," declared the exile, "for there the wind is never quiet. Often it blows a regular hurricane."

"That's what we want!" cried Tom. He had made several models of the air glider, changing them as he found out his errors, and at last he had hit on the right shape and size.

Midway of the big glider, on which work was