crossings, turn-tables, and canal drawbridges, and for many other purposes. There is, moreover, in use on some of the branch railways, a system of what is known as "Key Interlocking" for siding points, which merits a particular, description, as it is both simple and effectual, and might prove valuable in the working of temporary or Military railways. By means of this system, while the large outlay required for laying down rods to work the points is avoided, all the security of interlocking is retained. A simple frame of levers is provided for working the signals, with a number of keys. If a certain siding is required to be used, the key which applies to that siding is withdrawn, and this has the effect of mechanically locking all signals for the road with which the siding connects. The key is then carried to the siding, and with it the points are unlocked, but when they are once opened the key cannot be withdrawn until they are closed and locked again, and the signals, of course, cannot be lowered until the key has been carried back and inserted in the lever frame, so that the security is complete.
During the interval between 1859, when the first interlocking apparatus was fixed on the London and North-Western Railway, and 1873, when, as before stated, considerable progress had been made towards interlocking the whole of the system, many alterations and improvements in the method, both of signalling and interlocking, had from time to time been introduced with the inevitable result that the signal plant and apparatus in use had come to be of very mixed types, causing great difficulty and unnecessary expense in maintenance and repairs.
This state of things led to the introduction of a new
