We were all waiting expectantly for Craig to speak.  Evidently the intruder was now mounting the stairs to the art-gallery.

“Actually I can hear the light of the stars shining in through that wonderful plate glass skylight of yours, Mr. Spencer,” he went on.  “A few moments ago when the moon shone through I could hear it, like the rumble of a passing cart.  I knew it was the moon both because I could see that it must be shining in and because I recognised the sound.  The sun would thunder like a passing express-train if it were daytime now.  I can distinguish a shadow passing between the optophone and the light.  A hand moved across in front of it would give a purring sound, and a glimpse out of a window in daylight would sound like a cinematograph reeling off a film.

“Ah, there he is.”  Craig was listening with intense excitement now.  “Our intruder has entered the art-gallery.  He is flashing his electric light cane about at various objects, reconnoitring.  No doubt if I were expert enough and had had time to study it, I could tell you by the sound just what he is looking at.”

“Craig,” I interrupted, this time very excited myself, “the buzzing from the high-frequency current is getting lower and lower.”

“By George, then, there is another of them,” he replied.  “I’m not surprised.  Keep a sharp watch.  Tell me the moment the buzzing increases again.”

Spencer could scarcely control his impatience.  It had been a long time since he had been a mere spectator, and he did not seem to relish being held in check by anybody.

“Now that you are sure the vandal is there,” he cut in, his cigar out in his excitement, “can’t we make a dash over there and get him before he has a chance to do any more damage?  He might be destroying thousands of dollars’ worth of stuff while we are waiting here.”

“And he could destroy the whole collection, building and all, including ourselves into the bargain, if he heard so much as a whisper from us,” added Kennedy firmly.

“That second person has left the storeroom, Craig,” I put in.  “The buzzing has returned again full force.”

Kennedy tore the wireless receiver from his ear.  “Here, Walter, never mind about that electric detective any more, then.  Take the optophone.  Describe minutely to me just exactly what you hear.”

He had taken from his pocket a small metal ball.  I seized the receiver from him and fitted it to my ear.  It took me several instants to accustom my ears to the new sounds, but they were plain enough, and I shouted my impressions of their variations.  Kennedy was busy at the window over the heavy package, from which he had torn the wrapping.  His back was toward us, and we could not see what he was doing.

A terrific din sounded in my ears, almost splitting my ear-drums.  It was as though I had been suddenly hurled into a magnified cave of the winds and a cataract mightier than Niagara was thundering at me.  It was so painful that I cried out in surprise and involuntarily dropped the receiver to the floor.