Again, it is well known that, next to collisions, the greatest source of danger to railroad trains is due to broken tracks. It is, of course, apparent that tracks may at any time be broken by accident, as by earth-slides, derailment or the fracture of rails. This danger has to be otherwise provided for; the block has nothing to do with it further than to prevent a train delayed by any such break from being run into by any following train. The broken track which the perfect block should give notice of is that where the break is a necessary incident to the regular operation of the road. It is these breaks which, both in America and elsewhere, are the fruitful source of the great majority of railroad accidents, and draw-bridges and switches, or facing points as they are termed in the English reports, are most prominent among them. Wherever there is a switch, the chances are that in the course of time there will be an accident. Four matters connected with train movement have now been specified, in regard to which some provision is either necessary or highly desirable: these are rear collisions, tracks broken at draw-bridges or at switches, highway grade crossings, and the notification of agents and passengers at stations. The effort in America, somewhat in advance of that crowded condition of the lines which makes the adoption of something a measure of present necessity, has been directed towards the invention of an automatic system which at one and the same time should cover all the dangers and provide for all the needs which have been referred to, eliminating the risks incident to human forgetfulness, drowsiness and weakness of nerves. Can reliable automatic provision thus be made?--The English authorities are of opinion that it cannot. They insist that "if automatic arrangements be adopted, however suitable they may be to the duties which they have to perform, they should in all cases be used as additions to, and not as substitutions for, safety machinery worked by competent signal-men. The signal-man should be bound to exercise his observation, care and judgment, and to act thereon; and the machine, as far as possible, be such that if he attempts to go wrong it shall check him."