seded the disc signals of the earlier period. The: old semaphores showed three positions, "all right," "slacken speed," and "danger," the arms being actuated by the pointsman by means of a lever on the post. Up to this time, although the necessity must have existed, as it does to-day, for a driver to be warned as to the state of the line some time before actually reaching the point of obstruction, where he was required to stop, no attempt seems to have been made to meet the want; but at length accident proved once more the parent of design, for in the year 1846 a pointsman, who had to attend to two station signals, placed some little distance apart, in order to save himself the trouble of walking to and fro between them, procured some wire, which he attached to the levers of the signals, using a broken iron chair as a counter-weight, and by this simple expedient found himself able to work both signals without leaving his hut. Thus was demonstrated the possibility of working a signal at a distance, and this man's primitive contrivance doubtless hastened the introduction of distant signals, the use of which rapidly became universal. "Starting signals" were the development of a later period, the necessity for them having only been brought about by the application of the Block-Telegraph system some years after.
The next step in advance was the adoption, in a very elementary form, of the principle of interlocking, and this seems to have been forced upon the attention of railway engineers at a very early period, since, as early as 1843, we find that the levers for working the signals at a junction were provided with a simple mechanical device to prevent the main-line signal being lowered at the same time as that for the branch line; but, at that
