In the case of a highway crossing at a level, the electric bell under Hall's system is placed at the crossing, giving notice of the approaching train from the moment it is within half a mile until it passes; so that, where this appliance is in use, accidents can happen only through the gross carelessness of those using the highway. When the electric bell is silent there is no train within half a mile and the crossing is safe; it is not safe while the bell is ringing. As it now stands the law usually provides that the prescribed signals, either bell or whistle, shall be given from the locomotive as it approaches the highway, and at a fixed distance from it. The signal, therefore, is given at a distance of several hundred yards, more or less, from the point of danger. The electric system improves on this by placing the signal directly at the point of danger,--the traveller approaches the bell, instead of the bell approaching the traveller. At any point of crossing which is really dangerous,--that is at any crossing where trees or cuttings or buildings mask the railroad from the highway,--this distinction is vital. In the one case notice of the unseen danger must be given and cannot be unobserved; in the other case whether it is really given or not may depend on the condition of the atmosphere or the direction of the wind. Usually, however, in New England the level crossings of the more crowded thoroughfares, perhaps one in ten of the whole number, are protected by gates or flag-men. Under similar circumstances in Great Britain there is an electric connection between a bell in the cabin of the gate-keeper and the nearest signal boxes of the block system on each side of the crossing, so that due notice is given of the approach of trains from either direction. In this country it has heretofore been the custom to warn gate-keepers by the locomotive whistle, to the intense annoyance of all persons dwelling near the crossing, or to make them depend for notice on their own eyes. Under the Hall system, however, the gate-keeper is automatically signalled to be on the look out, if he is attending to his duty; or, if he is neglecting it, the electric bell in some degree supplies his place, without releasing the corporation from its liability. In America the heavy fogs of England are almost unknown, and the brilliant head lights, heavy bells and shrill high whistles in use on the locomotives would at night, it might be supposed, give ample notice to the most careless of an approaching train. Continually recurring experience shows, however, that this is not the case. Under these circumstances the electric bell at the crossing becomes not only a matter of justice almost to the employé who is stationed there, but a watchman over him.