Craig was waiting for me there with MacLeod, and at once opened the packages. From them he took a thin steel rod, which he set up in the center of the open space. To it he attached a frame and to the frame what looked like four reversed megaphones. Attached to the frame, which was tubular, was an oak box with a little arrangement of hard rubber and metal which fitted into the ears. For some time Kennedy's face wore a set, far-away expression, as if he were studying something.
"The explosions seem always to occur in the middle of the afternoon," observed MacLeod, fidgeting apprehensively.
Kennedy motioned petulantly for silence. Then suddenly he pulled the tubes out of his ears and gazed about sharply.
"There's something in the air!" he cried. "I can hear it!"
MacLeod and I strained our eyes. There was nothing visible.
"This is an anti-aircraft listening-post, such as the French use," explained Craig, hurriedly. "Between the horns and the microphone in the box you can catch the hum of an engine, even when it is muffled. If there's an aeroplane or a Zeppelin about, this thing would locate it."
Still, there was nothing that we could see, though now the sound was just perceptible to the ear if one strained his attention a bit. I listened. It was plain in the detector; yet nothing was visible. What strange power could it be that we could not see or feel in broad daylight?
Just then came a low rumbling, and then a terrific roar from the direction of the plant. We swung about in time to see a huge cloud of debris lifted literally into the air above the tree-tops and dropped to earth again. The silence that succeeded the explosion was eloquent. The phantom destroyer had delivered his blow again.
"The distillery—where we make the denatured alcohol!" cried MacLeod, gazing with tense face as from other buildings, we could see men pouring forth, panic-stricken, and the silence was punctured by shouts. Kennedy bent over his detector.
"That same mysterious buzzing," he muttered, "only fainter."