Accordingly we three went down cellar.  There, Elaine found the light switch and turned it.  Eagerly I hunted about for a mark.  There, in some rubbish that had not yet been carted away, was a small china plate.  I set it up on a small shelf across the room and took the gun.  But Elaine playfully wrenched it from my hand.

“No,” she insisted, “it was sent to me.  Let me try it first.”

Reluctantly I consented.

“Switch off the light, Walter, please,” she directed, standing a few paces from the plate.

I did so.  In the darkness Elaine pointed the gun and pulled a little ratchet.  Instantly a spot of light showed on the wall.  She moved the revolver and the spot of light moved with it.  As it rested on a little decorative figure in the center of the plate, she pulled the trigger.  The gun exploded with a report, deafening, in the confined cellar.

I switched on the light and we ran forward.  There was the plate—­ smashed into a hundred bits.  The bullet had struck exactly in the centre of the little bull’s-eye of light.

“Splendid,” cried Elaine enthusiastically, as we looked at each other in surprise.

Though none of us guessed it, half an hour before, in the seclusion of his yacht, Woodward’s friend, Professor Arnold, had been standing with the long barrelled gun in his hand, adjusting the tube which ran beneath the barrel.

In one hand he held the gun; in the other was a piece of paper.  As he brought the paper before the muzzle and pressed a ratchet by gripping the revolver handle, a distinct light appeared on the paper, thrown out from the tube under the barrel.

Having adjusted the tube and sighted it, Arnold wrote a hasty note on another piece of paper and inserted it into the barrel of the gun, with the end sticking out just a bit.  Then he wrapped the whole thing up in a box, rang a bell, and handed the package to a servant with explicit instructions as to its delivery to the right person and only to that person.

Down in the submarine harbor, Del Mar was in conference with his board of strategy and advice, laying the plan for the attack on America.

“Ever since we have been at work,” he remarked, “Elaine Dodge has been busy hindering and frustrating us.  That girl must go!”

Before him, on the table, he placed a square package.  “It must stop,” he added ominously, tapping the package.

“But how?” asked one of the men.  “We’ve done our best.”

“This is a bomb,” replied Del Mar, continuing to tap the package.  “When our man—­let me see, X had better do it,—­arrives, have him look in the secret cavern by the landing-place.  There I will leave it.  I want him to put it in her house to-night.”

He handed the bomb to one of his men who took it gingerly.  Then with a few more words of admonition, he took up his diving helmet and left the headquarters, followed by the man.