"Well," I said as we walked, my arm linked in hers. "First, as you know, a Zeppelin is constructed mostly of aluminium, its stays and practically all its rigid parts are of that metal except some of light steel. It consists of a number of ballonets filled with highly inflammable gas, and around those ballonets are ribs of aluminium and steel. There must be joints in these ribs, and over those joints we have now proved that we can create sparks from a considerable distance. From the ballonets there is a constant leakage of gas, therefore if we charge the aluminium and steel so that they spark wherever there is the slightest gap we shall ignite that escaping gas and cause the whole airship to explode with terrific force. Do I explain it clearly?" "Quite, Claude," was her slow, thoughtful reply. "I see now in what direction all these wonderful and patient experiments have been made. To-night you have certainly produced sparks." "And ere long I hope we shall increase our range, and be able to do without half the current and all its consequent paraphernalia," was my confident reply. "I'm certain," I said, "as certain as we are walking here together, that we have at last established a sound means of protecting Great Britain against Zeppelin raids." "I hope you have, dear," Roseye replied. "Oh! what a great thing it will be for the country. You and Teddy will deserve monuments--if you really can succeed." "We _shall_ succeed, darling--with your assistance. I'm confident of that!" "I--how can I help?" "In many ways. You've already assisted us enormously," I said. "Teddy was only saying so to-day," and I gripped her arm more tightly, as we turned the corner and approached the shed where Ashton was, we knew, awaiting us. "Splendid, my dear fellow!" I cried as we re-entered. "Sparking beautifully, all over--like fireworks!" "Pretty dangerous fireworks!" my friend remarked. "I cut off the current just now." "Yes," I said. "Why?" "Well, do you know, old chap, I thought I heard somebody about!" he replied. "Even with the dynamo running I fancied I overheard voices. Therefore I cut off at once, and went outside to see. Strangers seemed to be somewhere at the back." "Did you find anyone?" Roseye asked. "Nobody--yet I'm quite certain I heard voices," he insisted. "Some of the men from the market-garden perhaps," I remarked. "I don't think so," was Teddy's reply. "Why not?" I demanded in surprise. "Well--because what I heard--and I tell you, Claude, I heard it quite distinctly--was a sudden exclamation of surprise." "Surprise!" "Yes. As though somebody had made an unexpected discovery," Teddy said. "I had just been watching the effect on the pole through your glasses, and had returned inside when I heard an exclamation, followed by some quick words of surprise that I could not catch. It was a man's voice." "Surely there could not be anybody else watching the sparking upon the pole!" I exclaimed in quick apprehension.