“It was found necessary to establish a means of communication between the inside of Schwartzberg and the outside. Letters or written messages would not do; signal lights might be seen; secret meetings were almost impossible, for one could not often steal successfully in and out of a place watched as this one is.”
“No,” agreed Scanlon, “it couldn’t be depended on. And neither could the vaults be used as a meeting place. For the door to them is the most watched thing in the house.”
“A way must be had,” said Ashton-Kirk, “and one that must be silent and secret. This man, Alva, as Fuller’s report tells, is an able physicist, and so the method hit upon of bridging this difficulty must be his.” He looked at them as though asking their particular attention. “The eye,” said he, “is capable of vision only up to a certain point. It will follow an object going up into the air; then the object will disappear; it is ‘out of sight.’ However, though the object can’t be seen, it is still there, still going upward.
“You’ve heard the yell of the siren, a thing used upon the seagoing ships?” he proceeded. “You’ve heard its shriek mount and mount, getting higher and higher, and finally you ceased to hear it? But it had not stopped. It was still going on, only it had reached a pitch so high that it was out of ear-shot. It was only when it began to fall and had reached the point where you had lost it, that you began to hear it once more.”
Mr. Scanlon drew down one corner of his mouth and blinked a great number of times.
“What do you know about that!” said he.
“Perhaps the world’s greatest authority upon sound,” Ashton-Kirk went on as he took some notes from his pocket-book, “is the German, Helmholtz. In his book ‘On the Sensations of Tone’ he says:
“‘The simple partial tones contained in a composite mass of musical tones produce peculiar mechanical effects in nature, altogether independent of the human ear and its sensations, and altogether independent of merely theoretical considerations. These effects consequently give a peculiar objective significance to this peculiar method of analyzing vibrational forms.’
“Then,” continued Ashton-Kirk, “this master of sound goes on to speak of the phenomenon of sympathetic resonance. He says on this point: ‘When, for example, the strings of two violins are in exact unison, and one string is bowed, the other will begin to vibrate.’ And in another place: ‘Gently touch one of the keys of a pianoforte without striking the string, so as to raise the damper only, and then sing a note of the corresponding pitch, forcibly directing the voice against the strings of the instrument. On ceasing to sing the note will be echoed back from the piano. It is easy to discover that this echo is caused by the string which is in unison with the note, for directly the hand is removed from the key, and the damper is allowed to fall, the echo ceases.’
“We see, in the case of the siren, and in other things, that some tones are so high that they are not heard. Also we see, by Helmholtz, that when a string keyed to a certain tone is struck, another string, keyed to the same tone, will at once take up the sound, or vibration——”