time, no attempt was made to extend the locking to the levers which actuated the points. In the following year Mr. C. F. Whitworth patented a scheme for locking points and signals on the ground by means of locks worked by wire, but there was no suggestion for concentrating the levers. The apparatus was complicated and difficult to work, and apparently it was never brought to bear, but there is no doubt that the proposal contained the germ of the interlocking system as we have it to-day.
In 1856 a successful attempt was made by Mr. John Saxby, at the Bricklayers' Arms Junction (London), to concentrate and interlock the levers working both points and signals, and although the apparatus employed was crude as compared with the perfect mechanism now in use, it represented the earliest practical application of the principle of interlocking. In 1859 the first interlocking frame was fixed on the London and North Western Railway, at Willesden Junction, by Mr. Austin Chambers, who patented his arrangement in 1860, and from this point the interlocking of the London and North Western system proceeded rapidly; for thirteen years later, in 1873, it is recorded that 13,000 interlocked levers were in use on that railway.
At this point it may be worth while briefly to describe the meaning and application of the different kinds of signals and locking appliances in use upon the principal railways at the present time.
The form of signal most generally adopted is the "Semaphore," which is no doubt familiar to most of my readers, and consists of a timber or iron pole varying in dimensions according to circumstances, but sometimes as much as 70 feet high, with an arm about 5 feet long,
