be played upon. In the organ, a touch of the finger serves to depress a key, for the movement has only to admit a puff of air to certain pipes; but here the keys require a strong and steady pull, for they have to move ponderous point bars, or broad semaphore arms, and their movements have to be conveyed round many corners and over considerable distances. In both cases the mode of communicating motion is the same, the two mechanisms differing only in size and strength; and thus far the organ and the signal instrument exactly correspond. Now, however, we come to a point in which they differ toto cœlo. A performer on the organ can touch any keys he pleases in any order or in any number; he can 'discourse most eloquent music,' or he can rend the ears of his audience by abominable discord. Not so the signalman. Concord he can produce at will, but discord is utterly beyond his powers. He cannot open the points to one line and at the same time give a safety signal to a line which crosses it; and the points must be properly set, close home to the stock, or fixed rail, or the signal for a train to pass cannot possibly be given, and the least obstruction occurring to prevent the full and true opening or closing of the points is at once discovered, even with connecting rods of the greatest length practicable. Moreover, while a train is actually travelling through the points, it is itself master of the situation; not even the signalman can, either intentionally or inadvertently, change their position or disturb them until the whole train is safely passed. When he gives a clear signal for a main line, he cannot open a point crossing to it; when he gives a clear signal for a crossing he must show danger for all the
