"Have any spies been lurking about here to-night?" asked Roseye very anxiously. "That is my firm conviction," was Teddy's reply to her. "I believe that there have been two strangers here. One was, perhaps, gazing through his glasses at the pole and, seeing in the darkness the sparking over the insulators set in the steel guys, ejaculated the natural expression of surprise that I overheard. But they got away noiselessly, and all my search failed to discover them." "Well--we must be very wary, my dear Teddy," I repeated. "They must not get at this secret of ours, otherwise from the gondola of a Zeppelin they will be able to use the invisible force against any of our aeroplanes in a stronger and greater degree than we could ever hope to do it. Then we ourselves would be destroyed by the secret power we have invented." "They shall never know the secret from me," was my friend's fierce reply. "Only we three know it--while Theed has, of course, learnt something. That could, not be helped." "We must not forget the words I read out to you the other day from the _Berliner Tageblatt_," I replied. "That paper said: `The fires and devastation caused by our Zeppelin squadron in England represented a victory greater and more important than could be achieved in a single battle.' That," I added, "is the triumphant boast of Major Moraht, Germany's most prominent military critic." "Yes, and it went further," exclaimed Teddy, turning to Roseye. "The paper declared that if the Germans were as brutal as they were accused of being, their naval airship squadron could long ago, in memory of the Baralong, have set London afire at all four of her corners." "That's just what we intend to prevent," I declared very emphatically. "That is what, notwithstanding the efforts of prowling strangers who are seeking to know in what direction our experiments are being conducted, we intend to achieve. To-night, Roseye, we have made one great and astounding discovery--a discovery which has placed within our hands a power which Germany, with all her science and investigation, little dreams. We now know the true secret which will eventually prove the undoing of the Kaiser and his barbarous hordes." "Yes, dear," was my well-beloved's reply. "At all hazards, no spy of Germany must be allowed to wrest this secret from us." "But they are clever--devilishly cunning and entirely unscrupulous. The Invisible Hand, well provided with money, lurks everywhere, ready to grasp what it can in the interest of our octopus enemies," I declared warningly. "Therefore let us be ever on the alert--ever watchful and mindful, in order to avert the relentless talons with which this unknown and Invisible Hand is furnished." CHAPTER FIVE. THE RAID ON LONDON. It was the night of the fourteenth of October, in the year 1915.