Laurie, Ted, and Mr. Hazen were in the shack on a Saturday afternoon not long after the adventure on the river. A hard shower had driven them ashore and forced them to scramble into the shelter of the camp at the water's edge. How the rain pelted down on the low roof! It seemed as if an army were bombarding the little hut! Within doors, however, all was tight, warm, and cosy and on the hearth before a roaring fire the damp coats were drying.  
In the meantime the two boys and the young tutor had dragged out some coils of wire and a pair of amateur telephone transmitters which Ted had concocted while in school and for amusement were trying to run from one end of the room to the other a miniature telephone. Thus far their attempts had not been successful and Ted was becoming impatient.
 
"We got quite a fair result at the laboratory after the things were adjusted," commented he. "I don't see why we can't work the same stunt here."
 
"I'm afraid we haven't put time enough into it yet," replied Mr. Hazen. "Don't you remember how long Alexander Graham Bell, the inventor of the telephone, experimented before he got results?"
 
Laurie, who was busy shortening a bit of wire, glanced up with interest.
 
"I can't for the life of me understand how he knew what he wanted to do, can you?" he mused. "Think of starting out to make something perfectly new—a machine for which you had no pattern! I can imagine working out improvements on something already on the market. But to produce something nobody had ever seen before—that beats me! How did he ever get the idea in the first place?"
 
The tutor smiled.
 
"Mr. Bell did not set out to make a telephone, Laurie," he answered. "What he was aiming to do was to perfect a harmonic telegraph, a scheme to which he had been devoting a good deal of his time. He and his father had studied carefully the miracle of speech—how the sounds of the human voice were produced and carried to others—and as a result of this training Mr. Bell had become an expert teacher of the deaf. He was also professor of Vocal Physiology at Boston University where he had courses in lip reading, or a system of visible speech, which his father had evolved. This work kept him busy through the day so whatever experimenting he did with sounds and their vibrations had to be done at night."
 
"So he stole time for electrical work, too, did he?" observed Ted.
 
"I'm afraid that his interest in sound vibration caused him a sorry loss of sleep," said the tutor. "But certainly his later results were worth the amount of rest he sacrificed. One of the first agencies he employed to work upon was a piano. Have you ever tried singing a note into this instrument when the sustaining pedal is depressed? Do it some time and notice what happens. You will find that the string tuned to the pitch of your voice will start vibrating while all the others remain quiet. You can even go farther and try the experiment of uttering several different pitches, if you want to, and the corresponding strings will give back your notes, each one singling out its own particular vibration from the air. Now the results reached in these experiments with the piano strings meant a great deal more to Alexander Graham Bell than they would have meant to you or to me. In the first place, his training had given him a very acute ear; and in the next place, he was able to see in the facts presented a significance which an unskilled listener would not have detected. He found that this law of sympathetic vibration could be repeated electrically and, if desired, from a distance by means of electromagnets placed under a group of piano strings; and if afterward a circuit was made by connecting the magnets with an electric battery, you immediately had the same singing of the keys and a similar searching of each for its own pitch."
 
"I'd like to try that trick some time," exclaimed Ted, leaning forward eagerly.
 
"So should I!" echoed Laurie.
 
"I think we could quite easily make the experiment if Laurie's mother would not object to our rigging up an attachment to her piano," Mr. Hazen responded.
 
"Oh, Mater wouldn't mind," answered Laurie confidently. "She never minds anything I want to do."
 
"I know she is a very long-suffering person," smiled the tutor. "Do you recall the white mice you had once, Laurie, and how they got loose and ran all over the house?"
 
"And the chameleons! And the baby alligator!" chuckled Laurie. "Mother did get her back up over that alligator. She didn't like meeting him in the hall unexpectedly. But she wouldn't mind a thing that wasn't alive."
 
"You call an electric wire dead then," said Ted with irony.
 
"Well, no—not precisely," grinned Laurie. "Still I'm certain Mater would be less scared of it than she would of a mouse, even if the wire could kill her and the mouse couldn't."
 
"Let's return to Mr. Bell and his piano strings," Ted remarked, after the laughter had subsided.
 
Mr. Hazen's brow contracted thoughtfully and in his leisurely fashion he presently replied:
 
"You can see, can't you, that if an interrupter caused the electric current to be made and broken at intervals, the number of times it interrupted per second would, for example, correspond to the rate of vibration in one of the strings? In other words, that would be the only string that would answer. Now if you sang into the piano, you would have the rhythmic impulse that set the piano strings vibrating coming directly through the air, while with the battery the impulse would come through the wire and the electromagnets instead. In each case, however, the principle involved would be the same."
 
"I can see that," said Ted quickly. "Can't you, Laurie?"
 
His chum nodded.
 
"Now," continued Mr. Hazen, "just as it was possible to start two or more different notes of the piano echoing varying pitches, so it is possible to have several sets of these make-and-break or intermittent currents start their corresponding strings to answering. In this way one could send several messages at once, each message being toned to a different pitch. All that would be necessary would be to have differently keyed interrupters. This was the principle of the harmonic telegraph at which Mr. Bell was toiling outside the hours of his regular work and through which he hoped to make himself rich and famous. His intention was to break up the various sounds into the dots and dashes of the Morse code and make one wire do what it had previously taken several wires to perform."
 
"It seems simple enough," speculated Laurie.
 
"It was not so simple to carry out," declared Mr. Hazen. "Of course, as I told you, Mr. Bell could not give his entire time to it. He had his teaching both at Boston University and elsewhere to do. Nor was he wholly free at the Saunders's, with whom he boarded at Salem, for he was helping the Saunders's nephew, who was deaf, to study."
 
"And in return poor Mrs. Saunders had to offer up her piano for experiments, I suppose," Ted observed.
 
"Well, perhaps at first—but not for long," was Mr. Hazen's reply. "Mr. Bell ............