It is frequently urged that the celerity with which mechanical gates can be swung round and closed across the public road, is in itself a source of danger, and that persons preparing to cross the line might be struck by a moving gate, unless they received a distinct warning that such closing was about to take place. There is no doubt persons have been struck by such gates when closing across the road, and heavy claims for injuries have been decreed against railway companies, who were unable to prove that the man in charge had called out or given warning before moving the gates. To ensure that due and undeniable warning shall always be given, a firm of signal-makers have patented an appliance by which a powerful electric gong, fixed on the top of a tall post close to the gates, is sounded automatically by the gate machinery itself, and before the gates actually commence to move. As previously described, the pulling over of the lever to lower the gate-stops is the first operation to be performed whenever it is necessary to change the position of the gates, and it is the pulling over of this lever which actuates the apparatus, by bringing two electric points into contact, and thus starting the ringing of the gong or alarm. The gong continues to sound until the gates are moved over, the gate-stops raised, and the stop-lever put forward again into its normal position. The arrangement is very simple and very effective, and being purely automatic must work as regularly as the stop-lever. The tone and volume of the gong can be varied to suit circumstances. The public soon become familiar with its sound, and recognize its meaning. [Illustration: Fig. 509, 510] Figs. 509 and 510 give sketch plan and elevation of a set of mechanical gates for a public road level crossing on a double line of railway. The signal-cabin should be placed within a few yards of the gates, to enable the man in charge to have a good view of the persons and vehicles passing over the roadway. The underground gearing for working the gates and stops, must be protected by iron or wooden casing. The swinging portion of the wicket gates is closed, and held by a separate lever. The gates shown on the sketch are for a crossing on the square, but they can be equally well arranged for an oblique 338 crossing, and of widths to suit the locality.