be black, the up signals red, the down signals blue and the distant signals green. The row of levers thus presents a diversified pattern to the eye, which is readily caught by the parti-coloured groups, and, having once got the key, distinguishes quickly and correctly between their different classes.
"On examining the levers somewhat more closely, we perceive that many of them have numbers painted on their sides, not one number only, but in some cases half a dozen or more; and one naturally asks what can be their meaning. These numbers involve the whole secret of the safety which is secured by the mechanism, as will be readily understood on examining the principles on which it has been devised.
"The keys and pedals of an organ, as every one knows, command numerous valves admitting air from a wind-chest to the pipes which it is desired to sound. The key-boards are sometimes double or triple, and are occasionally arranged so that the performer sits with his back to the instrument. The pipes are generally spread over a large space, and sets of them are sometimes enclosed in separate chambers. There thus arises considerable complexity in the mechanism by which the several keys are made to operate on their respective air-valves. Nevertheless, by means of rods, cranks, and levers, such a connection is effected that, on depressing a C key, not one C pipe only, but it may be twenty C pipes are made to sound, in whatever part of the instrument those pipes may be situated. And so it is with the point and signal levers. The whole row may be considered to form a key-board, every key of which is connected by suitable cranks and rods to some one of the points and semaphores which have to
