Another thing struck him in a very forcible manner: on either side of the gallery were hung rectangular steel plates, measuring roughly ten feet by six, and painted in a medley of colours. It was like the so-called "invisible" colouring on the guns and gun-shields of British fortifications, the idea being that at a distance the various hues would blend and form a neutral tint. But in this instance the metal plates were hinged at the top edge. They could be raised by means of levers, and thus form a V-shaped covering to the gallery, presumably as a protection from high-angle fire or from explosives dropped from a hostile airship.
But Hamerton had little leisure to observe these things. The sergeant waited until he had received the password for the Sandinsel portion of the fortress, and then gave the order to march.
The enclosed way seemed interminable. For one thing, it never ran in a straight line for more than twenty yards at one time. Here and there other galleries branched both to left and right--the majority to the right. In places huge armoured bridges crossed the concealed way. On one a travelling crane, electrically propelled, was in the act of crossing.
The whole place seemed alive, judging by the sounds. The Sub could hear the sharp rattle of pneumatic riveters, the peculiar scroop of electric drills, and the thud of ponderous hammers, punctuated by the deafening hiss of compressed air. From a greater distance came the monotonous grinding sounds of a fleet of dredgers at work. There was no mistaking that: Hamerton had seen and heard dredgers at work in the principal naval ports at home. The groaning, rasping noise as the heavily-laden buckets come jerkily up the "ladder", the succession of dull thuds as bucket after bucket throws its contents of mud, sand, and larger stones down the shoot into the hopper, could not be mistaken for anything else.
Hamerton was puzzled. A fleet of dredgers--or even a solitary one--could be heard miles off. Yet during his enforced detention at Heligoland he had never heard the faintest sound that suggested operations of that nature.
"Well, it's either one of two things," he thought. "Either the cliffs of Heligoland possess strange acoustic properties, and deflect the sound entirely, or else these vessels have started work to-night for the first time--at least, since I set foot upon the island."