It is not, however, alone in connection with terminal stations and junctions that the interlocking apparatus is of value. It is also the scientific substitute for the law or regulation compelling trains to stop as a measure of precaution when they approach grade-crossings or draw-bridges. It is difficult indeed to pass from the consideration of this fine result of science and to speak with patience of the existing American substitute for it. If the former is a feature in the block system, the latter is a signal example of the block-head system. As a device to avoid danger it is a standing disgrace to American ingenuity; and, fortunately, as stopping is compatible only with a very light traffic, so soon as the passage of trains becomes incessant a substitute for it has got to be devised. In this country, as in England, that substitute will be found in the interlocking apparatus. By means of it the draw-bridge, for instance, can be so connected with the danger signals--which may, if desired, be gates closing across the railroad tracks--that the one cannot be opened except by closing the other. This is the method adopted in Great Britain not only at draws in bridges, but frequently also in the case of gates at level road crossings. It has already been noticed that in Great Britain accidents at draws in bridges seem to be unknown. Certainly not one has been reported during the last nine years. The security afforded in this case by interlocking would, indeed, seem to be absolute; as, if the apparatus is out of order, either the gates or the bridge would be closed, and could not be opened until it was repaired. So also as respects the grade-crossing of one railroad by another. Bringing all trains to a complete stop when approaching these crossings is a precaution quite generally observed in America, either as a matter of statute law or running regulation; and yet during the six years 1873-8 no less than 104 collisions were reported at these crossings. In Great Britain during the nine years 1870-8 but nine cases of accidents of this description were reported, and in both the years 1877 and 1878 under the head of "Accidents or Collisions on Level Crossings of Railways," the chief inspector of the Board of Trade tersely stated that,--"No accident was inquired into under this head.[21]" The interlocking system there affords the most perfect protection which can be devised against a most dangerous practice in railroad construction to which Americans are almost recklessly addicted. It is, also, matter of daily experience that the interlocking system does afford a perfect practical safeguard in this case. Every junction of a branch with a double track road involves a grade-crossing, and a grade-crossing of the most dangerous character. On the Metropolitan Elevated railroad of New York, at 53d street, there is one of these junctions, where, all day long, trains are crossing at grade at the rate of some twenty miles an hour.