The annals of railroad accidents are full of cases of "rear-end collision," as it is termed.[11] Their frequency may almost be accepted as a very accurate gauge of the pressure of traffic on any given system of lines, and because of them the companies are continually compelled to adopt new and more intricate systems of operation. At first, on almost all roads, trains follow each other at such great intervals that no precaution at all, other than flags and lanterns, are found necessary. Then comes a succeeding period when an interval of time between following trains is provided for, through a system of signals which at given points indicate danger during a certain number of minutes after the passage of every train. Then, presently, the alarming frequency of rear collisions demonstrates the inadequacy of this system, and a new one has to be devised, which, through the aid of electricity, secures between the trains an interval of space as well as of time. This last is known as the "block-system," of which so much has of late years been heard. [11] In the nine years 1870-8, besides those which occurred and were not deemed of sufficient importance to demand special inquiry, 86 cases of accidents of this description were investigated by the inspecting officers of the English Board of Trade and reported upon in detail. In America, 732 cases were reported as occurring during the six years 1874-8, and 138 cases in 1878 alone. The block-system is so important a feature in the modern operation of railroads, and in its present stage of development it illustrates so strikingly the difference between the European and the American methods, that more particular reference will have presently to be made to it.[12] For the present it is enough to say that rear-end collisions occur notwithstanding all the precautions implied in a thoroughly perfected "block-system." There was such a case on the Metropolitan road, in the very heart of London, on the 29th of August, 1873. It happened in a tunnel. A train was stalled there, and an unfortunate signal officer in a moment of flurry gave "line clear" and sent another train directly into it. [12] Chapter XVII.