The action of this rock plate is peculiar.  It is pivoted at the rear end, not to a fixed point, but to a short crank arm, the bearing for which is inclosed in the small box shown.  As the first wheel of a train which is approaching in the desired direction (from the right in the engraving) touches it, it will be seen that it must not only depress it, but produce a slight forward motion, causing a corresponding rotary motion in the rock shaft which actuates the apparatus.  On the other hand, when a train is approaching from the other direction, or has already passed the crossing, its wheels strike first the curved plate to the left of the illustration, and by means of the peculiar link connections shown, depress the rock plate so as to clear the wheels before the wheels touch it, but the depression is directly vertical, so that it does not give any horizontal motion to it, which would have the effect of actuating the rock shaft.  Consequently, trains pass over the apparatus in one direction without having any effect upon it whatever, the different point at which the same force is applied to the rock plate giving the latter an entirely different motion.

[Illustration:  FIG. 2.—­MAGNETO-ELECTRIC CROSSING SIGNAL]

The slight rotary motion which is in this way communicated to the rock shaft, when a train is approaching in the right direction, compresses a spring inside the case.  As each wheel passes off the rock plate, the reaction of the spring throws it up again to its former position, giving additional speed to the gearing within, which is set in motion at the passage of the first wheel, and operates the electric “generator.”  The spring is really the motive power of the alarm.  A small but heavy fly-wheel is connected with the apparatus, the top of which is just visible in the engraving, which serves to store up power to run the “generator,” which is nothing more than a small dynamo, for the necessary number of seconds after the rear of the train has passed.  The dynamo dispenses with all need for batteries, and reduces the work of maintenance to occasionally refilling the oil-cups and noticing if any part has been broken.

A suitable wire circuit is provided, commencing at the generator with insulated and protected wire, and continued with ordinary telegraph wire, which can be strung on telegraph poles or trees leading to the electric gong, Fig. 2, which rings as long as the armature revolves.  It is a simple matter so to proportion the mechanism for the required distance and speed that the revolutions of the armature and the ringing of the gong shall continue until the train reaches the crossing; and as each wheel acts upon the apparatus, the more wheels there are in the train the longer the bell will ring, a very convenient property, since the slowest trains have nearly always the most wheels.  The practical limits to the ringing of the gong are that it will stop sounding after the head of the train has passed the crossing and before or very soon after