Half a dozen motorcycles sprang into crackling life; there was the unmistakable din of a powerful aeroplane engine, which, with no muffler, is noisy enough to wake the dead.

Then came the whirring of its propeller.

They were sure that if they only dared to raise their heads, they would see the machine rising near by.
But there was more to follow that was just as inexplicable.

The motorcycles chugged away; then three automobiles started.

Their engines roared for a moment before they subsided to the ordered, steady hum of a smooth running motor.