Here is where that little thermometer plays its heroic part. It is called a thermostat, and it is so arranged that as the heat increases, the mercury in the bulb slowly rises to what is known as the danger-point. When the heat reaches that point the mercury sets an electric current going. At once the bell on the indicator where the sentry outside the Captain's door stands rings violently. The sentry hurries to it and sees a fire-alarm from a certain compartment, and he hastily awakes the Captain. The latter presses a button, perhaps without getting out of bed or up from his chair, and instantly there rings the general fire-alarm throughout the ship, and every man on board is called to quarters. For a few seconds it is time of immense confusion and noise. Great gongs are ringing in various parts of the ship. Men are hurrying half dressed, if it be in the night, here and there, and there is much shouting in the giving and passing of orders. In a twinkling, however, order prevails, and through the aid of that little automatic thermometer the ship and the lives of those on board are saved. This thermostat is an insignificant-looking affair--a mere trifle in the ship's construction--but see what an important thing it really is. These instruments are used in many buildings on land, but nowhere are they of such importance as when placed in coal-bunkers or on the outside walls of magazines in war-ships. Another dial on the indicator where the sentry paces also plays an important part in war-ships. It is called the water-alarm. All modern war-ships have what are known as double bottoms. They are built to prevent the ship from sinking or from becoming flooded in the chief compartments of the vessel below the water-line. Sometimes a ship may scrape along the top of an unknown rock or reef, or may strike some obstruction floating unseen beneath the waves. No one on board may feel the shock, especially if it is a light one. The water may rush into one of these double bottoms, and although the ship may be safe from sinking, the danger in time may be most grave. As the water fills the compartment which has been broken, a little piece of wood rises with it, and finally, when it reaches a certain height, it too establishes an electric current, and the alarm rings outside the cabin door of the Captain. Again the alarm sounds through the ship, and if possible, the break is mended temporarily, and the pumps set going to clear the compartment of the water. [Illustration: A TORPEDO BURSTING AN OUTER COMPARTMENT.]