[Illustration: DIAGRAM III. If you try to hold your trench with less men, as in this diagram, you would only cover a portion of the front; you would leave a gap in it, between X and Y, through which the trench would be carried.] It is evident, I repeat, that in practice there are needed to hold trenches a great many more men than this. You must allow for your wastage, for the difference in ability and coolness of different men, for the relieving of the men at regular and fairly short intervals, and in general, it will be found that a line of trenches is not successfully held with less than 3000 men to a mile. The Germans are now holding in the West a line of trenches 500 miles long with something like 4000 men to a mile; so the best work in the war would seem to have been done by a portion of the British contingent in front of Ypres when, apparently, a body only 1500 men to the mile, and those I understand, dismounted cavalry, successfully held some three miles of trenches for several days. It is apparent, then, that when you are considering a line of trenches you must consider them as a series of sections, to defend each of which sections a certain minimum is required. Thus we may consider the line A-B in Diagram IV as consisting of nine sections, as numbered, and each section as requiring a certain minimum unit of men, say a thousand. If any section has less than its proper minimum the whole line fails, for that section will be carried and the cord will be broken. [Illustration: DIAGRAM IV. The line of trenches A-B may consist of nine sections, to defend each of which 1000 men are required. If any section has less than its proper minimum the whole line fails.] [Illustration: DIAGRAMS V and VI. Suppose by killed, wounded and prisoners the nine sections dwindle to six, the line A-B can no longer be held. The six remaining sections would have to group themselves as above, and in either case there would be a bad gap. What then can the general in command of this dwindled force do?-- (_See Diagram VII overleaf._)] Now look back at the first diagram; there you have the line A-B, and there are nine units just able to hold it. Suppose by killed and prisoners and wounded and disease the nine dwindle to six, then the line A-B can no longer be held. It means in practice that the six remaining would have to be grouped as in Diagram V or as in Diagram VI, and in any case there would be a bad gap, double or single, through which the enemy pressing from E would pierce. What can the general in command of the defence do when his force has thus dwindled? [Illustration: --DIAGRAM VII. The defender has no choice but to fall back on shorter lines, such as F-G, which his remaining six units can just hold. If the six dwindle to four he must again fall back to a yet shorter line, C-D.]