Ten minutes later they landed on the firm, hard snow and lost no time in getting things in shape to spend the night where they were; for it was unlikely that repairs could be effected in time for them to fly back to the camp before dark. The canvas curtains at the sides of the aeroplane's body were drawn up, forming a snug tent. The stove was set going and soup and canned meats and vegetables warmed and eaten by the light of a lantern.
In the meantime Frank had discovered that the breakdown had been caused by a defect in the ignition apparatus which it would take some time to repair. Both he and Harry went to work on it after supper, however, and by midnight they had it adjusted.
They were just preparing to turn in, the professor and Billy having wrapped themselves in their blankets some time before, when a sudden sound, breaking on the stillness of the Antarctic night, made them pause. Both boys strained their ears intently and the sound came once more.
This time there was no mistaking it.
It was the same sound to which Rastus had called Frank's attention the night they were on watch outside the hut.
Pulling the curtain open, the boys gazed out, determined to unravel the mystery once and for all. The night was perfectly still except for the buzzing noise, and a bright moon showed them the snow lying white and undisturbed about them.
The sound did not proceed from the ground, that was evident, but from the air. The atmosphere seemed filled with it.
"What can it be?" exclaimed Harry.
"Look—look there!" shouted Frank, at the same instant clutching his brother's arm in his excitement.
Both boys gazed upward and as they did so a dark, shadowy form passed above them far overhead. For an instant a brilliant light gleamed from it and then it vanished, going steadily eastward with the strange thrumming sound growing fainter as it receded.