“Certainly a very promising one for me,” put in Kennedy.  “How has this machine been improved?  I’ve seen the old ones, but this is the first time I’ve seen this.  How does it work?”

“Well,” explained Gaines, with just a touch of pride, “you see, for studying blood flow in the extremities, I slip this cuff over my arm, we’ll say.  Suppose it is the effect of pain I want to study.  Just jab that needle in my other arm.  Don’t mind.  It’s in the interest of science.  See, when I winced then, the plethysmograph recorded it.  It smarts a bit and I’m trying to imagine it smarts worse.  You’ll see how pain affects blood flow.”

As he watched the indicator, Kennedy asked one question after another about the working of the machine, and the manner in which the modern psychologist was studying every emotion.

“By the way, Walter,” he interrupted, glancing at his watch, “call up and see if they’ve started with Errol and the rest yet.  Don’t stop, Gaines.  I must understand this thing before they get here.  It’s just the thing I want.”

“I should be glad to let you have it, then,” replied Gaines.

“I think I’ll need something new with these people,” went on Kennedy.  “Why, do you know what I’ve discovered?”

“No, but I hope it’s something I can add to my report?”

“Perhaps.  We’ll see.  In the first place, I found that digitalis had been put in Marchant’s tea.”

“They’ll be here directly,” I reported from the telephone, hanging it up and joining them again.

“It couldn’t have been an accident, as Karatoff said,” went on Kennedy, rapidly.  “The drug increased the blood pressure of Marchant, who was already suffering from hardening of the arteries.  In short, it is my belief that the episode of the rubber dagger was deliberately planned, an elaborate scheme to get Marchant out of the way.  No one else seems to have noticed it, but those slips of paper on which we all wrote have disappeared.  At the worst, it would look like an accident, Karatoff would be blamed, and—­” There was a noise outside as the car pulled up.

“Here, let me take this off before any of them see it,” whispered Gaines, removing the cuff, just as the door opened and Errol and Karatoff, Carita Belleville and Edith Gaines entered.

Before even a word of greeting passed, Kennedy stepped forward.  “It was not an accident,” he repeated.  “It was a deliberately planned, apparently safe means of revenge on Marchant, the lover of Mrs. Gaines.  Without your new plethysmograph, Gaines, you might have thrown it on an innocent person!”

X

THE SUBMARINE MINE

“Here’s the bullet.  What I want you to do, Professor Kennedy, is to catch the crank who fired it.”

Capt.  Lansing Marlowe, head of the new American Shipbuilding Trust, had summoned us in haste to the Belleclaire and had met us in his suite with his daughter Marjorie.  Only a glance was needed to see that it was she, far more than her father, who was worried.