Figure 17 is a reproduction from a postal card dated September, 1875, and issued at that time. It illustrates means for operating a positive safety signal in the cab of a locomotive when the track ahead is clear and safe, the operative current passing through the rails from the distant end of the track section upon which the train is entering. This system is elaborated in Robinson's British patent of August 29, 1879, where it is shown operatively applied to a single track in such a manner as to operate a signal on a locomotive approaching from either direction, the operative current coming from the opposite end of the section--no line wires being used. It is not thought necessary, therefore, to more fully describe the system here. In General The scriptural injunction, "Prove all things, hold fast that which is good," is the key note of scientific progress. He who would discover truth must not accept anything _because_ it is popularly accepted, or reject anything _because_ it is popularly rejected; nor must he regard anything as impossible _because_ never heretofore accomplished, although perhaps attempted by the most able scientists. While giving full weight to principles and laws demonstrated and verified by original investigations, he must bear in mind that those principles and laws may be capable of various combinations and interpretations; that the popular interpretation may not be capable of general application, and if not, it must be erroneous. In short, he must enter upon his investigations systematically, independently and untrammeled by prejudice. These remarks apply to electrical science with great force at the present time. Those who enter this field to advantage should be men of culture, of theoretical knowledge, and eminently practical. These facts are illustrated by the efforts heretofore put forth in Europe and the United States to develop systems of rail signaling. Such efforts, in the early days, appear to have been exerted principally by theorists whose propositions and complications prove them to be not only ignorant of some of the fundamental principles of electrical science, but also, some of them, extremely unpractical. That the efforts in this direction may be fairly understood we will direct attention to a few of the systems of rail signaling proposed,--those which have elicited most attention--giving outline illustrations of some of the circuits which form their bases, and pointing out their defects and merits. Early Rail Systems So far as we have knowledge, the idea of using the rails as conductors for electric signaling purposes was first suggested in an English patent of 1848. This was merely a suggestion, however, and no attempt was made to describe any specific method of using the rails for the purpose.