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 soon abandoned piano strings and in their place resorted to flat
strips of springy steel, keying them to different pitches by varying
their length. One end of these strips he fastened to a pole of an
electromagnet and the other he extended over the other pole and left
free."

"And the current