If it is important to have efficient means of stopping trains, it is scarcely less important to have timely information of the need of stopping them. To give such information is the function of signals, which, among safety appliances, must stand next after brakes. Signals fall naturally into two great classes: Those which protect points of danger and govern the movements of engines in yards, and those which keep an interval of space between two trains running on one track. For the protection of switches, crossings, junctions, and the like, signals in immense variety have been used, and, unfortunately, are still used; but in the last ten or fifteen years the semaphore signal has become the general standard in the United States, as it long has been in England. This consists of a board, called the blade or arm, pivoted on the post, and back of the pivot is a heavy casting which carries a colored glass lens, either green or red. On the post is hung a lantern. The danger position is with the blade horizontal. In this position the lens is in front of the lamp, and the light shows red or green, as the case may be. The safety position is with the blade hanging about sixty degrees from the horizontal. In this position the light of the lantern shows white. Red is the universal danger color, and green the color of caution. Therefore, a semaphore signal at a point of danger shows by day a blade painted red, with the end of the blade cut square. At night it shows a red light. At a position some distance from the point of actual danger, but where it is desirable to warn an engine-runner that he is likely to find the danger signal against him, a caution signal is placed. This is a semaphore blade painted green, with the end notched in a V-shape, or, as it is called, a fish-tail. At night this signal shows a green light. There is nothing very remarkable about a piece of board arranged to wag up and down on a pin stuck through a post, but it is wonderful how much of good brains and good breath have been expended in getting these boards to wag harmoniously, and in getting railroad officers to understand that a plain board, having two possible positions, is a better signal than any more complicated form. [Illustration: Section of Saxby & Farmer Interlocking Machine. (Showing two levers and locking mechanism. _A_ is normal, _B_ is reversed.)]