In time of battle, however, this water-alarm may tell a more important story. Perhaps a torpedo from the enemy has struck the ship, and a gaping wound has been torn not only in the outside bottom of the ship, but through the inner hull as well. Instantly the news reaches the Captain through the water-alarm. The Captain simply presses a button, and at once not only does the general alarm ring, but the "siren" whistle on the ship is set to shrieking most horribly. Those siren whistles are seldom heard either in port or at sea. They begin their noise with a low moan, and run up to an awful shriek, with a thin, ear-piercing note that is almost unendurable. The siren may be blown by electricity as easily as an electric door-bell may be rung. When the siren is heard it is a signal throughout the ship to close all the water-tight doors in the various compartments, and thus confine the inrushing waters to a limited space. If only one or two compartments are torn open by the torpedo, the ship may be saved from sinking and a great tragedy of war may be averted. When the siren howls, however, there is such a scurrying on shipboard as is never seen at other times. Nearly every compartment has men in it at work. The alarms and the whistles are their only warning, except, perhaps, the shouts of their companions. A mighty rush is made to get out of some of these compartments. No time must be lost, and there can be no waiting for a man to escape. If shut in, he may be drowned. It is a question of his life or that of the ship and the lives of the rest of the crew, and there is only one way to answer that question. These water-alarms are used in many large buildings in connection with the fire-alarm, but one can see how much more important they are on ships, especially on war-ships, than on land. They are a most simple contrivance, and, like the thermostat, mere trifles; but they may turn the tide in a naval battle, and directly or remotely settle the fate of a nation.