The apparatus which has been described, though a great improvement on anything which had preceded it, was still but a clumsy affair, and naturally the friction of the levers on the locks was so great that they soon became worn, and when worn they could not be relied upon to move the switch-points with the necessary accuracy. The new appliance of safety had, therefore, as is often the case, introduced a new and very considerable danger of its own. The signals and switches, it was true, could no longer disagree, but the points themselves were sometimes not properly set, or, owing to the great exertion required to work it, the interlocking gear was strained. This difficulty resulted in the next and last improvement, which was a genuine triumph of mechanical ingenuity. To insure the proper length of stroke being made in moving the lever--that is to make it certain in each case that the switch points were brought into exactly the proper position--two notches were provided in the slot, or quadrant, as it is called, in which the lever moved, and, when it was thrown squarely home, and not until then, a spring catch caught in one or other of these notches. This spring was worked by a clasp at the handle of the lever, and the whole was called the spring catch-rod. By a singularly ingenious contrivance, the process of interlocking was transferred from the action of the levers and the keys to these spring catch-rods, which were made to work upon each other, and thus to become the medium through which the whole process is effected. The result of this improvement was that, as the switchman cannot move any lever until the spring-catch rod is fastened, except for a particular movement, he cannot, do what he will, even begin any other movement than that one, as the levers cannot be started. On the other hand, it may be said that, by means of this improvement, the mere "intention of the signal-man to move any lever, expressed by his grasping the lever and so raising the spring catch-rod, independently of his putting his intention in force, actuates all the necessary locking.[19]"