telepathy and suggestion together, as it were, you have more than the elements of what I call the Z Ray? I feel myself that I have more to go on in making my hypothesis than the inventor of the steam engine had in making his hypothesis when he saw the lid of the kettle bobbing up and down. What do you say?” 

 Dr. Lewis made no answer. He was watching the growth of a new, unknown tree in his garden. 

 The doctor made no answer to Remnant’s question. For one thing, Remnant was profuse in his eloquence—he has been rigidly condensed in this history—and Lewis was tired of the sound of his voice. For another thing, he found the Z Ray theory almost too extravagant to be bearable, wild enough to tear patience to tatters. And then as the tedious argument continued Lewis became conscious that there was something strange about the night. 

 It was a dark summer night. The moon was old and faint, above the Dragon’s Head across the bay, and the air was very still. It was so still that Lewis had noted that not a leaf stirred on the very tip of a high tree that stood out against the sky; and yet he knew that he was listening to some sound that he could not determine or define. It was not the wind in the leaves, it was not the gentle wash of the water of the sea against the rocks; that latter sound he could distinguish quite easily. But there was something else. It was scarcely a sound; it was as if the air itself trembled and fluttered, as the air trembles in a church when they open the great pedal pipes of the organ. 

 The doctor listened intently. It was not an illusion, the sound was not in his own head, as he had suspected for a moment; but for the life of him he could not make out whence it came or what it was. He gazed down into the night over the terraces of his garden, now sweet with the scent of the flowers of the night; tried to peer over the tree-tops across the sea towards the Dragon’s Head. It struck him suddenly that this strange fluttering vibration of the air might be the noise of a distant aeroplane or airship; there was not the usual droning hum, but this sound might be caused by a new type of engine. A new type of engine? Possibly it was an enemy airship; their range, it had been said, was getting longer; and Lewis was just going to call Remnant’s attention to the sound, to its possible cause, and to the possible danger that might be hovering over them, when he saw something that caught his breath and his heart with wild amazement and a touch of terror. 

 He had been staring upward into the sky, and, about to 

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