I waited for a quarter of an hour, listening to those secret signals. Then I hurried to the telephone, and fortunately found Raymond at home. I told him what I had discovered, and urged him to take a taxi at once down to Sydenham and ascertain whether they were receiving signals there. This he promised to do, telling me he would 'phone me the result to the hotel at eight o'clock next morning. Therefore I returned to the factory, and through the long night-hours listened to their secret experiments. At eight next morning the telephone rang, and Ray briefly explained that Gessner, who had placed his apparatus upon the high flagstaff in his garden, had been receiving messages all night! "Have you seen anything of Fowler?" "No. But Hartmann has spent the night with Gessner, apparently watching his experiments. Couldn't you manage to watch your opportunity and get inside the factory somehow? I'll come north at noon, and we'll see what we can do." At five o'clock he stepped from the London express, and together we walked down to the Imperial Hotel, to which I had suddenly changed my quarters, feeling that I had been too long in the close vicinity of the spy Dubois. "It seems that they carry out their experiments at night," I explained. "For in the daytime the wireless apparatus is no longer in position. I see now why they engaged a builder to examine the chimney--in order to place a pulley with a wire rope in position at the top!" "But Gessner and Dubois are expert electricians, no doubt. Members of the Telegraphen-Abtheilung of the German army, most probably," remarked my friend. "And who is Fowler?" "A victim, I should say. He appears to be a most respectable man." "In any financial difficulty?" "Not that I can discover." "But why have they established this secret communication between Hull and London?" "That's just what we have to discover, my dear fellow," laughed Ray. "But if we are to get a peep inside the place it's evident we can only do so in the daytime. At night they are down there." "At early morning," I suggested, "after they have left." "Very well," he said; "we'll watch them to-night, and get in after they leave. I've brought a few necessaries in my bag--the set of housebreaking implements," he added, with a grin. "Well," I said, "neither of us know much about wireless telegraphy. Couldn't we get hold of an operator from one of the Wilson liners in dock, and take him along with us? A sailor is always an adventurer." Ray was struck with the idea, and by eight o'clock that evening we had enlisted the services of a smart young fellow, one of the operators in the Wilson American service, to whom, in strictest confidence, we related our suspicions. That night proved an exciting one. Fortunately for us it was cloudy, with rain, at intervals. Murphy, the wireless operator, listening under the wall declared that we were not mistaken. The men were sending messages in code.