[Illustration: RANGE-FINDERS AT WORK ON THE ENEMY FROM THE FORE AND MAIN TOPS.]
Nor will it do to fire at an enemy in the old hit-or-miss style that used to be followed. In the old days the gunner sighted the gun, made an allowance as best he could for the distance the ship would go before he fired, and when the roll or plunge of his own vessel was favorable he pulled a string, as he turned away his head, and took chances of hitting the other ship. He had to guess the distance largely. He first tried a certain range, and then another, until he gradually got the right distance, and then he fired, whenever he got the right chance. Those were the days of the old smooth-bore cannon; the days when a captain could make himself heard by a speaking-trumpet anywhere on deck, and almost anywhere below-deck by shouting down a hatchway. All that is changed now. The roar on a war-ship in battle is like that on a mountain-top encased in a violent thunder-storm. Then, too, ammunition is too costly and too limited nowadays to be wasted by experiments in finding the range of the enemy, and in taking chances on hitting him. One shot from one of these guns of modern times may win the battle by piercing the vitals of the enemy's ship, and not even the smallest chance must be taken to miss that target. It is for that reason that the wonderful aid of electricity has been called into use on war-ships in many devices. Probably the most wonderful of all these devices is the range-finder.
It isn't necessary for us to go deep into electrical science to understand how this instrument does its work. Electricity itself is a great mystery, and a puzzle in many ways to those who understand it best. Most of us have not been able to grasp its simpler puzzles until we studied them in college, and even then it would not be well for us to boast of what we knew. All we need to know, however, to understand the range-finder is a little problem in geometry. Most of you understand that if we have a triangle, and know the length of one side and the size of the angles at the ends of that side, we can figure exactly the length of the other two sides and the distance of the point where those two sides meet. Well, that explains just how the range-finder works. The exact distance between the two men looking through the telescopes is known and fixed. Each man looks, as we have seen, at the same point out in the distance on the enemy's ship. That point is where the two unknown sides of the triangle come together. There is no time to figure the distance. A tenth of a second in these days is too much time to be lost. We must use electricity to do our figuring. This is the way it is done: The telescopes are turned about on electric circuits; that is, they are attached to a metal circle charged with electricity. The wires from these circles run to a little dial down in the hold. The Wheatstone bridge or electrical balance system is used. That means that the resistance of a current through a wire through a given distance is measured finally by a dial on a marking-instrument.