frame," consisting of a row of levers by means of which the signalman actuates every pair of points and every signal under his control. By a mechanical contrivance, the points and signals are interlocked—that is to say, supposing, for example, that the signalman has moved a lever that opens a pair of points, to enable a train to come out of a siding on to the main line, the home and distant signals must be at "danger" to stop any train from approaching on the main line, and it is physically impossible for the signalman to lower them. The manner in which this is accomplished was described some years ago in the Engineer, and that description is so complete and lucid that probably it would be difficult to improve upon it. The writer says:—
"Mounting by an iron ladder to the signal platform, we enter the glass-house. One side of this building is occupied by a row of strong iron levers standing nearly upright from the floor, and placed at equal distances along one side of the apartment; on the opposite side are many electric telegraph instruments,and the rest of the width forms a gangway or passage from end to end for two stalwart and serious-looking men, whose time is entirely occupied in looking through the glass sides of their cell, and pulling this way or pushing that way some of the levers which are arranged before them. These levers work all the points and signals, and their number is found to correspond exactly with the number of points and signal levers outside. Every lever is numbered, and on the floor beside it there is fixed a brass plate, engraved with its name and use. Sets of them are also distinguished in a way that readily catches the eye, by being painted in strong colours. Thus, for example, all the point levers may
