Drew smiled good-naturedly. "It's a mystery to most people," he said, turning toward the windows and listening. "It's a bigger mystery to a woman than to a man," he added. "It's a good agent if properly used and kept within bounds. It brings back life as well as takes it. It creates and also destroys. No one knows what it is. All that we do know about it is its action on material substances--the power to transform mechanical energy into vibrations and then back again into mechanical energy." "Like a voice on a wire?" asked Loris. "Yes, Miss Stockbridge. The mechanical vibration of a diaphragm in a telephone transmitter is changed to electrical vibrations, passes along a wire and changes back to the same thing we had at the beginning. Cuthbert took advantage of this fact. All that was sent into the library was Morphy's voice on the wire. The wire left no trace. The voice actuated the diaphragm and at the same time the fine heating coil at the cap on the cartridge. The energy of the voice was sufficient to raise the temperature of the coil. This raise in temperature flashed some compound set in the wire. The flash started the fulminate of mercury in the cap. The cap exploded the smokeless powder. It was a series of steps each a little higher than the one below it." "Was there any other way of doing the same thing?" Nichols inquired, as he rose lankily and stood over Loris. "Yes!" declared Drew. "I can look back over what I found in the technical books about electricity and telephony and see several other ways for Cuthbert to accomplish the same result. The electrical pistol did not necessarily have to be actuated by the human voice." "How terrible!" Loris whispered, with her brow puckering. "Perhaps others will use the same idea to slay their enemies." "We'll keep it a close secret," the detective said. "It rests with us four, now. Outside of us, there is only Morphy who knows." "How else could the pistol be discharged?" "Two other ways that I see, Miss Stockbridge. It would be rather easy to arrange a little magnetic trigger in the receiver. This trigger could be actuated by an excess of current--say the connecting of a hundred and ten volt lighting circuit on the line. It might burn out the magnet wiring, but it would also release the trigger and fire the cartridge." "That's like a door-catch?" "Yes," said Drew. "Like a door-catch operated by a magnet or like the firing pin of a large cannon. They're not all operated by lanyards. Some work with push-buttons." Nichols passed his hand over his brow. "I know another way," he said, glancing down at Loris. "There is a way which is far cleverer than Cuthbert thought of. It could be done by a tuning-fork or reed."