It will now become necessary to glance in passing at the personal characteristics of Professor Eldridge. This man was in about his fortieth year, tall, spare, keenly intellectual in countenance, cold, possessed of an absolute reliance on the powers of science, beyond which his mental processes did not stray. His manner was distinguished by a stiff unbending formality; his expression by a glacial coldness of steel-gray eyes and a straight-line compression of thin lips; his dress by a precise and unvarying formalism, and his speech by a curious polysyllabic stiffness.

This latter idiosyncrasy would, in another, have seemed either priggish or facetiously intended. With Professor Eldridge it was merely a natural method of speech. Thus, arriving once at the stroke of the dinner hour, he replied to compliments on his punctuality by remarking:

"I have always considered punctuality a virtue when one is invited to partake of gratuitous nourishment."

Withal, his scientific attainments were not only undoubted, but so considerable as to have won for him against many odds the reputation of a great scientist. His specialty, if such it might be called, was scientific diagnosis. The exactness of scientific laws was so admirably duplicated by the exactitudes of his mind that he seemed able, by a bloodless and mechanical sympathy, to penetrate to the most obscure causes of the strangest events. It might be added that practically his only social ties were those with the Warfords, and that the only woman with whom he ever entered into conversation was Helen.

At sight of him Percy Darrow's lounging gait became accentuated to exaggeration.

"Hello, Prof!" he drawled. "On the job, I see. Good morning, Doctor," he greeted Knox. "What do you make of it?"

"I make of it that the Atlas Building will shortly be without tenants," replied the doctor; "me, for one."

Eldridge surveyed Darrow coldly through the glittering toric lenses of his glasses.

"The cause of these extraordinary phenomena is self-evident," he stated.

"You mean their nature, not their cause," replied Darrow. "In nature, they refer back to the interference with etheric and molecular vibrations. That," he added, "is a fact that every boy in the grammar-school physics class has figured out for himself. The cause is a different matter."

"I stand corrected," said Eldridge. "Such lapses in accuracy of statement are not usual with me, but may be considered as concomitant with unusual circumstances."

"Right-o!" agreed Darrow cheerfully. "Well, what about the causes?"

"That I will determine when I am satisfied that all the elements of the problem are in my hands."

"Right-o!" repeated Darrow. "Well, I'll bet you a new hat I'll land the cause before you do. Be a sport!"

"I never indulge in wagers," replied Eldridge.

"Well," said Darrow to Jack and Hallowell, "come on!"

Without waiting to see if he was followed, the young man again plunged into the black and clinging darkness.

"Get hold of my coat," his voice came to the others. "We're going to climb."

Accordingly they climbed, in silence, up many flights of stairs, through the cloying darkness. At last Darrow halted, turned sharp to the left, fumbled for a door, and entered a room.

"Simmons?" he said.

"Here!" came a voice.

"I thought you'd be on the job," said Darrow, with satisfaction. "How's your instrument? Going, eh? We are in the wireless offices," he told the others. "Sit down, if you can find chairs. We'll wait until the sun is shining brightly, love, before we really try to get down to business. In the meantime--"

"In the meantime--" repeated both Jack and Hallowell, in a breath. "Go on, my son," conceded the latter. "I bet we have the same idea."

"Well, I was going to say that I'm not in the grammar-school physics class, and I want to know what you meant by your remark to Eldridge," said Jack.

"That's my trouble," said Hallowell.

"It's simple enough," began Darrow. "We have had, first, a failure of all electricity; second, a failure of all sound; third, a failure of all light. The logical mind would therefore examine these things to see what they have in common. The answer simply jumps at you: _Vibration_. Electricity and light are vibrations in ether; sound is vibration in air or some solid. Therefore, whatever could absolutely stop vibration would necessarily stop electricity, light and sound."

"But," objected Jack, "if vibration were absolutely stopped, why wouldn't they all three be blotted out at once?"

"Because," explained Darrow, "the vibrations making these three phenomena are different in character. Sound is made by horizontal waves, for example, while electricity and light are made by transverse waves. Furthermore, the waves producing electricity and light differ in length. Now, it is conceivable that a condition which would interfere with horizontal waves would not interfere with transverse waves; or that a condition which would absolutely deaden waves two hundred and seventy ten-millionths of an inch long would have absolutely no effect on those one hundred and fifty-five ten-millionths of an inch long. Am I clear?"

"Sure!" came the voices of his audience.

"That much Eldridge and any other man trained in elementary science already knows. It is no secret."

"It hasn't been published," observed Hallowell grimly.

"Well, go to it! The task of the independent investigator, of which we are some, is now to discover, first, what are those conditions, and, second, what causes them. With the exception of Mr. Hallowell, we all know what this guiding power is."

"Don't get it," growled Simmons.

"Now, look here, Simmons, you are very loyal to McCarthy, for whatever reason, but your loyalty is misplaced. For one thing, your man has disappeared, and will not return. That last message scared him out. For another thing, we're going to need you in our campaign, the worst way."

"I'm from Copenhagen; you got to show me," said Simmons.

Darrow laughed softly.

"We'll show you, all right," said he. He sketched briefly for Hallowell's benefit the reasoning already followed out, and which it is therefore unnecessary to repeat here. "So now," he concluded, "we will consider this hypothesis: that these phenomena are caused by one man in control of a force capable of deadening vibrations in ether and solids within certain definite limits."

"Why do you limit it?" cried Hallowell.

"Because we have had but one manifestation at a time. If this Unknown were out really to frighten--which seems to be his intention--it woul............