Those intermittent sounds were that of the Morse code. They were exchanging signals with some other persons.
Gessner was absent. No doubt the corresponding station was at that house high upon Sydenham Hill to the south of London, two hundred miles distant!
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!"