The periscope, which forms such an important part of the submarine's equipment, is really very little more than a telescope. On the top there is a little mirror, or more probably a prism or three-cornered piece of glass which serves precisely the same purpose in that it reflects exactly as a mirror does. This is so placed that it throws the light from distant objects down the tube into the interior of the ship. In the tube are lenses very like those of an ordinary telescope and the light may be made to throw a picture upon a little table or screen or else can be viewed through another prism directly by the eye. In either case the periscope is just like an ordinary telescope set up vertically with a prism at the top so that it can "see" at right angles, and possibly another at the bottom so that the picture can be viewed at right angles to the direction of the tube. The latter is necessary only for the convenience of the observer, since otherwise he would have to be upon his back to look up the tube. The whole apparatus can be rotated mechanically and a scale forms a means of measuring the precise direction in which the prism or mirror is at any moment pointed. This is useful for measuring roughly the position of the "prey," and it may even be used as a rough means of getting the range. Another feature is the gyroscope compass, to which a passing reference has already been made. It is fairly well known that an object when spinning exhibits properties quite different from those which it possesses when still. A boy's top is a familiar illustration, for while spinning it will stand perfectly steady, supported only upon a tall peg with a sharp point, a pose which it will absolutely refuse to maintain when not spinning. Now fortunately for the present purpose it so happens that one of the peculiarities of the gyroscope or spinning-wheel is this: that if mounted in a certain way it persists in placing its axis in the same plane as that in which the axis of the earth lies. If you imagine for a moment a plane or flat surface of which the earth's axis forms a part you will see that wherever that plane cuts the surface of the earth will be a line in a north and south direction. Consequently, if any horizontal object has its axis in that same plane it, too, will always point north and south. A wheel, small but heavy, is therefore mounted with its axis supported horizontally upon a little metal raft floating in a trough of mercury and driven round at a very fast speed by a small electric motor fixed in it.