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 seemed like solid pieces of wood. Tom stood on
the ground near the levers which controlled the speed, and Ned watched
the scale.

"How much?" yelled the young inventor.

"Eight hundred."

Tom turned on a little more gasolene.

"How much?" he cried again.

"Ten hundred. That'll do!"

"No, I'm going to try for more."