"I was taking exercise," replied Hamerton.

"Exercise? Mein Gott, your form of exercise very remarkable is, Herr Smidt. Now you be sensible. I give you one more chance. If you behave not, then I report to Herr Major Kohn, and you will be put in far worse place."

The lieutenant and the surgeon walked out of the room, followed by the men; but half an hour later another jailer appeared with a pile of books, "for the use of Herr Smidt, with the compliments of Lieutenant Schaffer."

CHAPTER XVIII

The Errant Airship

The next day was warm and sunny, with hardly any wind. It would have been an ideal day for cricket, thought Hamerton, as he gazed through the window upon the deserted courtyard. It made confinement doubly hard and irksome.

Continuously the noise of machinery and the busy hum of workmen rose in the sultry air. The smell of petrol and occasionally wafts of hot mineral oil seemed to pervade the atmosphere.

Yet above the din could be heard the dull roar of the sea: the sullen breakers lashing themselves into masses of white foam upon the inner edges of the Hohe Brunnen shoal. It was a sure sign of a storm far out to sea.

The Sub was listless. In vain he tried to fix his mind upon the books the fair-haired lieutenant had lent him. It was all to no purpose. Again and again he threw down the volume and returned to the window to look upon a vista of paving stones and an almost blank wall.

At about three in the afternoon a shadow, travelling at a great pace, fell athwart the courtyard. It could not be a cloud, for the sky was of a deep-blue colour and destitute of any form of condensed vapour.