Next, there is the _angle indicator_, which is simply a _quadrant_—that is, a quarter of a circle—marked off into degrees and each degree into quarters. It has a needle which moves over the quadrant as the pilot turns the diving wheel and this indicates the number of degrees up or down the horizontal rudders have moved.
Another instrument is the _depth indicator_.
Then there is also an indicator which is merely a modified form of a carpenter’s _spirit_ level. This little device shows when the craft is running on an even keel and when it is running with its keel inclined. For instance, should the diving rudder fail to respond to the touch of the man at the wheel, the level would indicate it, as would also the depth indicator.
Other fittings are the levers and the valve controls, by means of which water can be let into the ballast tanks.
Now, before I explain how the submarine is made to dive and how the ballast tanks work, or, rather, the other way about, I want to tell you a little about the why and the wherefore of these very interesting operations.
=The Four States of the Submarine.=—A submarine has four states, or conditions, in which it exists in the water. These are (1) the _light_, or surface cruising condition; (2) the _awash_, or partly submerged condition; (3) the _submerged_ condition; and (4) the _totally submerged_ condition—all of which is clearly shown in the diagram Fig. 26.
[Illustration: FIG. 26. HOW THE SUBMARINE SUBMERGES OR THE FOUR STATES OF THE SUBMARINE.]
_The Light Condition._—The light, or cruising, condition is simply the position a submarine takes when she floats on the water, due to her own natural buoyancy, and it is exactly like that of any other ship.
While in this position the captain takes his observations from the deck, if there is no danger of being hit by the enemy; but if there is danger, he then makes his observations through the ports from the inside of the conning tower.
_The Awash Condition._—This condition is not as natural as the one just described, and it has to be done by means of the ballast tanks. These are located fore and aft and they can be connected with, or disconnected from, each other at will.
Now, when the captain wants to bring his submarine to the awash condition an _inlet valve_, or _Kingston valve_ as it is called, is opened and the sea-water then flows into these tanks, the amount and velocity of the inrushing water being, of course, under control at all times.