This extra weight destroys part of the buoyancy of the boat, and as she gets heavier she sinks until nothing but her conning tower remains above the water-line, the deck being awash. While in this condition observations are taken either from the ports in the conning tower or with the periscope, the object-glass of which has been lowered enough to become useful.
_The Submerged Condition._—In this condition the periscope alone remains above the water. It is had by allowing more water to flow into the ballast tanks, thus destroying a little more of the craft’s buoyancy, which makes her sink down until the conning tower is completely submerged. It is in this condition that the captain makes his observations with the periscope.
_The Totally Submerged Condition._—This is an exaggeration of the submerged condition, and it is had by letting still more water flow into the ballast tanks, thus sinking the submarine completely. The only way to steer the boat when it is in this condition is, of course, by means of the compass, for both the conning tower and the periscope are totally submerged.
But do not mistake the term _totally submerged_ to mean that the buoyancy of the submarine is _totally destroyed_; for such is not the case during any stage of its submergence. You can easily see that if the buoyancy were completely destroyed the submarine would then become a dead weight and sink to the bottom of the sea, never more to rise.
Instead, when the submarine is totally submerged she can, by what is known as her _reserve_, or _extra_ buoyancy, and about which you will read later on, come to the awash or the light condition in a few minutes by simply pumping the water out of the ballast tanks.
=How a Submarine Dives.=—Now let’s get back to the way a submarine dives. In the first place, let us suppose the boat is running in the light, or cruising, condition. An enemy ship is sighted and the captain of the undersea craft gives orders to clear the deck and close the hatches. Then he brings the boat from the cruising to the awash condition, which is done as we have just described.
Next, he gives the order to the man at the diving wheel to make the dive. He does not need to do this by word of mouth but he can use an electric indicator which points out the angle at which he wants the horizontal rudders set (see Fig. 27). Contrary, now, to what you might expect, a submarine cannot dive at any angle, but it must make a very shallow dive.