Just outside the door of the Captain's cabin, on every ship of the navy, there stands a sentry. He paces up and down for a distance of about ten feet. On one of the sides of the cabin is an electric indicator similar to those seen in the large hotels back of the clerk's desk. The sentry on the ship passes that indicator every time he paces from one end of his limited beat to the other. He cannot escape hearing its bell when it rings, and his eye at once sees whence the signal comes that is telegraphed to the Captain in time of emergency. That indicator is placed there so that, when necessary, there shall be instant communication with the Captain. Some of the dials tell stories of the utmost importance to the safety of the ship. They tell these stories automatically. Let us see how one of the most important of these dials may perhaps save the ship from destruction. Down in the coal-bunkers there is a little instrument attached to the side of each compartment that looks like a little thermometer. It is not more than four or five inches tall. It is simply a thermometer with an electric attachment. A fire has started in a coal-bunker, as happens sometimes on large steamships, through what is known as "spontaneous combustion." It may smoulder for several days and give no indication of its existence. At last it breaks into a flame. Some one has felt a hot deck through his shoes as he has walked along, or perchance has accidentally placed his hand on the iron-work of the compartment and found it blistering hot. Instantly the fire-alarm is rung, and if the fire is not too far advanced the ship may be saved. On war-ships, however, no such risk must be run. In the economy of space on such ships it frequently happens that these coal-bunkers are placed very near, and sometimes next to, the powder-magazines. A fire in the coal-bunkers would mean an awful explosion, the loss of the ship and hundreds of lives.