Sharp!" cried the lad. "Something has happened! We're heading for that tower!"

"Steer to one side!" called the balloonist.

Tom tried, but found that the helm had become jammed. The horizontal rudder would not work, and the craft was rushing nearer and nearer, every minute, to the pile of brick and mortar.

"We're going to have a collision!" shouted Tom. "Better shut off the power!"

The two propellers were whirling around so swiftly that they looked like blurs of light. Mr. Sharp came rushing forward, and Tom relinquished the steering wheel to him. In vain did the aeronaut try to change the course of the airship. Then, with a shout to Tom to disconnect the electric switch, the man turned off the power from the motor.

But it was too late. Straight at the tower rushed the Red Cloud, and, a moment later had hit it a glancing blow, smashing the forward propeller, and breaking off both blades. The nose of the aluminum gas container knocked off a few bricks from the tower, and then, the ship losing way, slowly settled to the flat roof of the building.

"We're smashed!" cried Tom, with something like despair in his voice.

"That's nothing! Don't worry! It might be worse! Not the first time I've had an accident. It's only one propeller, and I can easily make another," said Mr. Sharp, in his quick, jerky sentences. He had allowed some of the gas to escape from the container, making the ship less buoyant, so that it remained on the roof.

The aeronaut and Tom looked from the windows of the car, to note if any further damage had been done. They were just congratulating themselves that the rudder marked the extent, when, from a scuttle in the roof there came a procession of young ladies, led by an elderly matron, wearing spectacles and having a very determined, bristling air.

"Well, I must say, this is a very unceremonious proceeding!" exclaimed the spectacled woman. "Pray, gentlemen, to what are we indebted for this honor?"

"It was an accident, ma'am," replied Mr. Sharp, removing his hat, and bowing. "A mere accident!"

"Humph! I suppose it was an accident that the tower of this building was damaged, if not absolutely loosened at the foundations. You will have to pay the damages!" Then turning, and seeing about two score of young ladies behind her on the flat roof, each young lady eying with astonishment, not unmixed with admiration, the airship, the elderly one added: "Pupils! To your rooms at once! How dare you leave without permission?"