How can the submarine communicate with its own fleet? It has wireless telegraphy and also deep-water signals, but these do not work so well as might be desired. It has one other connection with the visible world as wonderful as anything described by Jules Verne in his _Twenty Thousand Leagues Under the Sea_, the periscope, or literally the “looker round.” I can not do better than describe it in the words of a naval officer, Count Ernest zu Reventlow: Roughly speaking, the apparatus consists in this: If the boat is under water and yet wishes to see what is going on above, it pushes up a long thin pipe until the surface is reached and a little beyond. At the farther end of this pipe is a contrivance with glass prisms, or mirrors and lenses. This throws down the image reflected from the surface of the water, through the vertical pipe, into the interior of the boat. The image is caught on a plate and the commander of the submarine, although he may be several yards under water, can see everything that is floating and happening on the surface and consequently can make his attack with the sole guidance of this image and steer the boat until it is at the right distance for firing the torpedo. It sounds like magic, and indeed the witches were not in it when it comes to the achievements of modern science. But Reventlow has to confess that in practise the periscope is not so wonderful as it sounds in theory. The splashing of the salt water, unless the sea be perfectly calm, which it seldom is, soon dims and even effaces the image. It was long before the inventors could bring the periscope to reflect more than a small section of the horizon, but that difficulty seems to have been overcome. It is possible, with map, clock and compass, to take reckonings and keep on a course even when deep down under water. Deeper than ninety feet the submarine seldom goes. It has found a new and unexpected enemy in the air-ship or aeroplane, for it is a well-known fact that from a height on a clear day, at least, you can see very far into the water. But what, one will ask, can the aeroplane do about it even if it sights a submarine far down beneath the surface? Projectiles would not be likely to do much damage. At the same time it can warn ships and can pursue and worry the submarine. That the latter is not a perfect instrument goes without saying; indeed, when it darts about blindly it becomes a menace to its own ships. Its arrangements are so complicated, too, with all the letting in and out of water, the diving and coming up, the changing of motors and providing artificial air that things are very apt to go wrong. The service is extremely exhausting for the men and extremely dangerous.