Dexterously, and without disturbing the position of the instrument, von Ruhle whipped off the covering. Although there were no visible signs that anything was taking place, both men knew that a beam of light, reflected from the distant lighthouse on Black Bull Head, was being directed seawards.
In silence the two men peered through the driving rain, von Ruhle making use of a pair of powerful night-glasses.
Suddenly, after an interval of almost five minutes, a faint pin-prick of light flickered from the surface of the sea.
Instantly Ramblethorne stepped a dozen paces to the right.
"I can see nothing from here," he announced in a low voice. "Can you?"
"Yes," replied his companion.
"Good: that's friend Schwalbe."
The doctor was right. From the deck of the unterseeboot a signalling apparatus similar to that employed by the spies was in use. By an ingenious automatic arrangement it projected a beam of light, derived from the same sources as that on St. Mena's Island, rigidly in a fixed direction, regardless of the "lift" of the submarine under the action of the waves.
For several minutes a rapid exchange of signals was maintained; then the two spies, folding up their apparatus, walked rapidly towards Main Beach Cove.
They had not long to wait before the faint sound of oars was borne to their ears.