He had not exaggerated. The set-up consisted of a whole bank of tubes, each one in its own shielding copper box. On a much-drilled horizontal panel, propped up on insulators, were half a score of delicate meters of one kind and another, with thin black fingers that pulsed and trembled.

Behind the panel was a tall cylinder wound with shining copper wire, and beside it another panel, upright, fairly bristling with k.n.o.bs, contact points, potentiometers, rheostats and switches. On the end of the table nearest the door was still another panel, the smallest of the lot, bearing only a series of jacks along one side, and in the center a switch with four contact points. A heavy, snaky cable led from this panel to the maze of apparatus further on.

"This is the control panel," explained Mercer. "The whole affair, you understand, is in laboratory form. Nothing a.s.sembled. Put the different antennae plug into these jacks. Like this."

He picked up a weird, hastily built contrivance composed of two semi-circular pieces of spring bra.s.s, crossed at right angles. On all four ends were bright silvery electrodes, three of them circular in shape, one of them elongated and slightly curved. With a quick, nervous gesture, Mercer fitted the thing to his head, so that the elongated electrode pressed against the back of his neck, extending a few inches down his spine. The other three circular electrodes rested on his forehead and either side of his head. From the center of the contrivance ran a heavy insulated cord, some ten feet in length, ending in a simple switchboard plug, which Mercer fitted into the uppermost of the three jacks.

"Now," he directed, "you put on this one"--he adjusted a second contrivance upon my head, smiling as I shrank from the contact of the cold metal on my skin--"and think!"

He moved the switch from the position marked "Off" to the second contact point, watching me intently, his dark eyes gleaming.

Carson entered, and Mercer gestured to him to wait. Very nice old chap, Carson, impressive even in his bathing suit. Mercer was mighty lucky to have a man like Carson....

Something seemed to tick suddenly, somewhere deep in my consciousness.

"Yes, that's very true: Carson is a most decent sort of chap." The words were not spoken. I did not _hear_ them, I _knew_ them. What--I glanced at Mercer, and he laughed aloud with pleasure and excitement.

"It worked!" he cried. "I received your thought regarding Carson, and then turned the switch so that you received my thought. And you did!"

Rather gingerly I removed the thing from my head and laid it on the table.

"It's wizardry, Mercer! If it will work as well on _her_...."

"It will, I know it will!--if we can get her to wear one of these,"

replied Mercer confidently. "I have only three of them; I had planned some three-cornered experiments with you, Carson, and myself. We'll leave Carson out of to-night's experiment, however, for we'll need him to operate this switch. You see, as it is now wired only one person transmits thoughts at a time. The other two receive. When the switch is on the first contact, Number One sends, and Numbers Two and Three receive. When the switch is on Number Two, then he sends thoughts, and Numbers One and Three receive them. And so on. I'll lengthen these leads so that we can run them out into the pool, and then we'll be ready.

Somehow we must induce her to wear one of these things, even if we have to use force. I'm sure the three of us can handle her."

"We should be able to," I smiled. She was such a slim, graceful, almost delicate little thing; the thought that three strong men might not be able to control her seemed almost amusing.

"You haven't seen her in action yet," said Mercer grimly, glancing up from his work of lengthening the cords that led from the antennae to the control panel. "And what's more, I hope you don't."

I watched him in silence as he spliced and securely taped the last connection.

"All set," he nodded. "Carson, will you operate the switch for us? I believe everything is functioning properly." He surveyed the panel of instruments hastily, a.s.suring himself that every reading was correct.

Then, with all three of the devices he called antennae in his hand, their leads plugged into the control panel, he led the way to the side of the pool.

The girl was strolling around the edge of the pool, feeling the smooth tile sides with her hands as we came into view, but as soon as she saw us she shot through the water to where we were standing.

It was the first time I had seen her move in this fashion. She seemed to propel herself with a sudden mighty thrust of her feet against the bottom; she darted through the water with the speed of an arrow, yet stopped as gently as though she had merely floated there.

As she looked up, her eyes unmistakably sought mine, and her smile seemed warm and inviting. She made again that strange little gesture of invitation.

With an effort I glanced at Mercer. There was something devilishly fascinating about the girl's great, dark, searching eyes.

"I'm going in," I said hoa.r.s.ely. "Hand me one of your head-set things when I reach for it." Before he could protest, I dived into the pool.

I headed directly towards the heavy bronze ladder that led to the bottom of the pool. I had two reasons in mind. I would need something to keep me under water, with my lungs full of air, and I could get out quickly if it were necessary. I had not forgotten the livid, jagged furrows in Mercer's side.

Quickly as I shot to the ladder she was there before me, a dim, wavering white shape, waiting.

I paused, holding to a rung of the ladder with one hand. She came closer, walking with the airy grace I had noted before, and my heart pounded against my ribs as she raised one long, slim arm towards me.

The hand dropped gently on my shoulder, pressed it as though in token of friendship. Perhaps, I thought quickly, this was, with her, a sign of greeting. I lifted my own arm and returned the salutation, if salutation it were, aware of a strange rising and falling sound, as of a distant humming, in my ears.

The sound ceased suddenly, on a rising note, as though of inquiry, and it dawned on me that I had heard the speech of this strange creature.

Before I could think of a course of action, my aching lungs reminded me of the need of air, and I released my hold on the ladder and let my body rise to the surface.

As my head broke the water, a hand, cold and strong as steel, closed around my ankle. I looked down. The girl was watching me, and there was no smile on her face now.

"All right!" I shouted across the pool to Mercer, who was watching anxiously. Then, filling my lungs with air again, I pulled myself, by means of the ladder, to the bottom of the pool. The restraining hand was removed instantly.

The strange creature thrust her face close to mine as my feet touched bottom, and for the first time I saw her features distinctly.

She was beautiful, but in a weird, unearthly sort of way. As I had already noticed, her eyes were of unusual size, and I saw now that they were an intense shade of blue, with a pupil of extraordinary proportion.

Her nose was well shaped, but the nostrils were slightly flattened, and the orifices were rather more elongated than I had ever seen before. The mouth was utterly fascinating, and her teeth, revealed by her engaging smile, were as perfect as it would be possible to imagine.

The great mane of hair which enveloped her was, as I have said, tawny in hue, and almost translucent, like the stems of some seaweeds I have seen. And as she raised one slim white hand to brush back some wisps that floated by her face, I saw distinctly the webs between her fingers. They were barely noticeable, for they were as transparent as the fins of a fish, but they were there, extending nearly to the last joint of each finger.

As her face came close to my own, I became aware of the humming, crooning sound I had heard before, louder this time. I could see, from the movement of her throat, that I had been correct in a.s.suming that she was attempting to speak with me. I smiled back at her and shook my head.

She seemed to understand, for the sound ceased, and she studied me with a little thoughtful frown, as though trying to figure out some other method of communication.

I pointed upward, for I was feeling the need for fresh air again, and slowly mounted the ladder. This time she did not grasp me, but watched me intently, as though understanding what I did, and the reasons for it.

"Bring one of your gadgets over here, Mercer," I called across the pool.

"I think I'm making progress."

"Good boy!" he cried, and came running with two of the antennae, the long insulated cords trailing behind him. Through the water the girl watched him, evident dislike in her eyes. She glanced at me with sudden suspicion as Mercer handed me the two instruments, but made no hostile move.

"You won't be able to stay in the water with her," explained Mercer rapidly. "The salt water would short the antennae, you see. Try to get her to wear one, and then you get your head out of water, and don yours.

And remember, she won't be able to communicate with us by words--we'll have to get her to convey her thoughts by means of mental pictures. I'll try to impress that on her. Understand?"

I nodded, and picked up one of the instruments. "Fire when ready, Gridley," I commented, and sank again to the bottom of the pool.

I touched the girl's head with one finger, and then pointed to my own head, trying to convey to her that she could get her thoughts to me.

Then I held up the antennae and placed it on my own head to show that it could not harm her.