It should be noted that the distant is only a _caution_ signal, whereas both home and starting are _stop_ signals. This means that when the driver sees the distant "on," he does not stop his train, but slackens speed, and prepares to stop at the home signal. He must, however, on no account pass either home or starting if they are at danger. In short, the distant merely warns the driver of what he may expect at the home. To prevent damage if a driver should overrun the home, it has been laid down that no train shall be allowed to pass the starting signal of one box unless the line is clear to a point at least a quarter of a mile beyond the home of the next box. That point is called the _standard clearing point_. Technically described, a _block_ is a length of line between the last stop signal worked from one signal-box and the first stop signal worked from the next signal-box in advance. [Illustration: FIG. 92.--Showing position of signals. Those at the top are "off."] INTERLOCKING SIGNALS. A signalman cannot lower or restore his signals to their normal positions in any order he likes. He is compelled to lower them as follows:--Starting and home; _then_ distant. And restore them--distant; _then_ starting and home. If a signalman were quite independent, he might, after the passage of a train, restore the home or starting, but forget all about the distant, so that the next train, which he wants to stop, would dash past the distant without warning and have to pull up suddenly when the home came in sight. But by a mechanical arrangement he is prevented from restoring the home or starting until the distant is at danger; and, _vice versa_, he cannot lower the last until the other two are off. This mechanism is called _locking gear_. LOOKING GEAR. There are many different types of locking gear in use. It is impossible to describe them all, or even to give particulars of an elaborate locking-frame of any one type. But if we confine ourselves to the simplest combination of a stud-locking apparatus, such as is used in small boxes on the Great Western Railway, the reader will get an insight into the general principles of these safety devices, as the same principles underlie them all. [Illustration: FIG. 93.--A signal lever and its connections. To move the lever, C is pressed towards B raising the catch-rod from its nick in the rack, G G G, guides; R R, anti-friction rollers; S, sockets for catch-rod to work in.]