But the spy did return, and, crouching in deepest shadow, became convinced that those whose business he was so anxious to discover had passed beneath the wharf. As he dared not attempt to follow them through the impenetrable gloom into which they had disappeared, he sought a hiding-place, and from it watched with infinite patience for them to again come forth.
They had, in the meantime, safely regained the snug living-room of the Mermaid, and reported all that had happened, to the Professor. Then Carl Baldwin unfolded his scheme for delaying the Spanish cruiser in port until after their departure.
As a result, the submarine boat was allowed to drift down the harbour with the ebbing tide, until she came abreast the great black hull of a man-of-war. Then she imperceptibly sank beneath the surface.
The watch officer of the Spanish cruiser, leaning on her after-rail and gazing musingly down into the dark waters sweeping seaward, speculated idly concerning the stream of phosphorescent light tailing out from under her counter, but thought of it only as a natural phenomenon. Had he known that it was caused by the motion of the Mermaid's propeller necessary to hold her in position against the stream while she hovered like a gigantic fish directly above the screw of his ship, how easily could he have won the promotion for which he longed. But he suspected nothing; and as Carl Baldwin, working from the diving chamber of the submarine craft, had succeeded in fastening one end of a short length of stout wire rope to the propeller blade, and shackling[!-- [Pg 18] --] the other to a ring-bolt in the massive rudder, the officer turned with a sigh and walked away.
On the following morning the Spanish spy, weary and cramped with his long vigil, was amazed to see an utter stranger emerge cautiously from beneath the wharf he had been watching, and walk quickly away. For a moment the spy was undecided as to whether he should follow this person or seek to discover where he had come from. Then choosing the former course, he followed Professor Rivers at a respectful distance, until he had the vast satisfaction of seeing him meet, near the custom-house, the captain of the tug that was avowedly bound for Havana.
There was a connection then between those who hid beneath the wharf and the suspected tow anchored in the harbour. Undoubtedly a store of contraband goods was concealed under the wharf, and an effort would be made to convey them on board the tug before she sailed. What a reward was in prospect for him could he but discover it!
A little later the spy, with two companions, all armed, occupied a skiff that made its way cautiously through the dark spaces beneath the wharf he had watched so long. Suddenly between them and the outer daylight two men appeared one after the other. Both slid down one of the piles supporting the pier and dropped into the water, or at least the exulting spy thought they did so as he hastily urged his boat in that direction.
To his amazement and disgust, when he reached the spot where they had disappeared, he could discover no trace of them. Neither was there a boat or a hiding-place into which they could have gone. The man was furious at being thus baffled, and uttered many a fierce Spanish oath. Finally, convinced that further search in that direction was fruitless, he pulled out into the harbour to watch the mysterious tow that still lay at anchor. As[!-- [Pg 19] --] he drew near to it he saw its captain come off from shore alone. Then the guard from one of the revenue cutters was withdrawn, anchors were lifted, and the tow began to move slowly down the channel. It was certain that no one save the captain had gone aboard, nor had any cargo been taken in except a few tons of carefully examined coal.