Page:Tom Swift and His Air Glider.djvu/18

8 "Come on back, and finish pumping up the tires," he shouted to Ned. "I'm going to stop her now, and then I'll give her the pressure test, and we'll take a trip."

Having cleared his eyes of smoke, Ned came back to his task, and this having been finished, Tom attached a heavy spring balance, or scales, to the rope that held the airship back from moving when her propellers were whirling about.

"How much pressure do you want?" asked Ned.

"I ought to get above twelve hundred with the way the motor is geared, but I'll go up with ten. Watch the needle for me."

It may be explained that when aeroplanes are tested on the earth the propellers are set in motion. This of course would send a craft whizzing over the ground, eventually to rise in the air, but for the fact that a rope, attached to the craft, and to some stationary object, holds it back.

Now if this rope is hooked to a spring balance, which in turn is made fast to the stationary object, the "thrust" of the propellers will be registered in pounds on the scale of the balance. Anywhere from five hundred to nine hundred pounds of thrust will take a monoplane or biplane up. But Tom wanted more than this.

Once more the motor coughed and spluttered, and the big blades whirled about so fast that they