Woodward looked at it, as Arnold had reversed it and his face lighted up.

“Harbor successfully mined,” he quoted in surprise.

“I’ll show you another thing about this radio detective of mine,” went on Arnold energetically.  “It’s not only a wave length measurer, but by a process of my own I can determine approximately the distance between the sending and the receiving points of a message.”

He attached another, smaller machine to the wireless detector.  In the face was a moving finger which swung over a dial marked off in miles from one upward.  As Arnold adjusted the new detector, the hand began to move slowly.  Woodward looked eagerly.  It did not move far, but came to rest above the figure “2.”

“Not so very far away, you see, Lieutenant,” remarked Arnold, pointing at the dial face.

He seized his glass and hurried to the deck, levelling it at the shore, leaning far over the rail in his eagerness.  As he swept the shore, he stopped suddenly.  There was a house-roof among the trees with a wireless aerial fastened to the chimney, but not quite concealed by the dense foliage.

“Look,” he cried to Woodward, with an exclamation of satisfaction, handing over the glass.

Woodward looked.  “A secret wireless station, all right,” he agreed, lowering the glass after a long look.

“We’d better get over there right away,” planned Arnold, leading the way to the ladder over the side of the yacht and calling to the sailor who had managed the little motor-boat to follow him.

Quickly they skimmed across to the shore.  “I think we’d better send to the Fort for some men,” considered Arnold as they landed.  “We may need reinforcements before we get through.”

Woodward nodded and Arnold hastily wrote a note on a rather large scrap of paper which he happened to have in his pocket.

“Take this to Colonel Swift at Fort Dale,” he directed the sailor.  “And hurry!”

The sailor loped off, half on a run, as Arnold and Woodward left down the shore, proceeding carefully.

At top speed, Arnold’s sailor made his way to Fort Dale and was directed by the sentry to Colonel Swift who was standing before the headquarters with several officers.

“A message from Lieutenant Woodward and Professor Arnold,” he announced, approaching the commanding officer and handing him the note.  Colonel Swift tore it open and read: 

Have located radio aerial in the woods along shore.  Please send squad of men with bearer.—­Arnold.

“You just left them?” queried the Colonel.

“Yes sir,” replied the sailor.  “We came ashore in his boat.  I don’t know exactly where they went but I know the direction and we can catch up with them easily if we hurry, sir.”

The colonel handed the note quickly to a cavalry officer beside him who read it, saluted at the orders that followed, turned and strode off, hastily stuffing the paper in his belt, as the sailor went, too.