A perfect knowledge of the gunboat’s whereabouts were had, even under water, by use of the periscope. This necessary part of every submarine’s equipment is a simple variation on the old camera obscura. A long tube, with a mirror at the top, projected above the surface of the water. It offers no more target than would the slender neck of a floating bottle, and would hardly attract any attention. Inside the tube are other mirrors, so arranged as to reflect a perfect picture of the waters above, upon a flat, white surface—something like the top of a desk—set in front of the helmsman in the conning-tower.
Of course, as it was at night, all that could be seen in the periscope reflector was the bright light on the gunboat’s masthead, but this served quite as well to locate her as if it had been daytime, and they could have seen the reflection of her whole outline.
They speedily drew ahead of the gunboat, which was not, apparently, making more than ten or twelve knots. Forging steadily forward, the submarine was maneuvered till she was directly across the gunboat’s bows, but some distance ahead.
“Is this about far enough, Strong, do you think?” asked Lieutenant Parry.
“I think so, sir,” nodded Ned gravely, “and now, if you think everything is ready for it, we’ll give them a whistle.”
“Go ahead,” nodded the officer.
Ned seized the compressed-air whistle’s lever and sent an eerie scream out over the waters. As its echoes died out they could see a sudden ray of light shoot upward from the dark form of the gunboat, as reflected in the periscope.
“They’ve heard the whistle, and are wondering what it is. There goes the searchlight!” cried Lieutenant Parry, as the fan-shaped ray hovered about for an instant, and then began to sweep the waters. “Dive quick, before they pick us up.”
Ned sprang to the wheel and jerked over the sinking controls. Instantly the little of the submarine that showed above the surface was completely submerged.
“How much does she draw, sir?” asked Ned, turning to the officer.