This comprises the line of fire-trenches, with covering fire-trenches, support trenches, support dugouts and reserve dugouts (in other words, it is an exact duplication of the front line system), far enough behind the front line that in the event of the first system being taken, the second line is ready to be taken up by the troops driven out of the front line, and receive the support of troops lying in brigade or divisional reserve. The distance of the second line behind the first is roughly a mile, and while the first line is held, operations of the second often provide useful accommodation for the machine guns and artillery, both for firing purposes, observation and shelter. The time available for siting and constructing second line system, and the freedom from all disturbances which bother the front line system, should result in its being impregnable. It is an ideal system provided those responsible for its siting and construction are thoroughly and practically experienced with first line conditions and profit by that experience and former mistakes. COMMUNICATION TRENCHES When siting communication trenches, two considerations come into conflict with one another; the desire for protection, and the necessity of rapid and easy transit. It is obvious that a C. T. is of vital importance to the firing line in the getting up of supporting troops, ammunition and stores rapidly and without undue fatigue. It is also obvious that it must give as much protection as possible to the troops using the C. T., but it must be recognized that the protection given must not interfere with rapid transit by making the C. T. too narrow. It must also be kept in mind that rapid transit in itself gives a certain amount of protection for the simple reason that the easier and quicker the transit, the less time are troops delayed in the C. T.’s in which protection is required, and if troops can cover the danger area in 10 minutes, they are plainly in danger for a less period than if it took 30 minutes to struggle over that same area. This rapid transit, only obtained by a dangerously wide communication trench from support or reserve lines and from dugouts, also makes it possible for less troops to be regularly on duty in the fire-trenches, which are always uncomfortable and dangerous, especially during a bombardment, and it also enables supports and reserves to be rushed up quickly when the occasion demands. It can be plainly seen now that a compromise must be effected between claims of protection and rapid transit, so that the size and shape of the C. T.’s will vary according to their distance from a danger area. Generally speaking, the nearer to danger, the more must the claims of rapid transit give way to those of protection.