The number of men per mile required for guarding a line of rail communication is declared by Captain John Bigelow, in his "Principles of Strategy" (Philadelphia, 1894), to be exceedingly variable, depending as it does upon the tactical features of the country and the temper of the inhabitants. According, he says, to the estimate of the Germans for the conditions of European warfare, the number will average about 1,000 men for every stretch of fifteen miles. At this rate an army sixty miles from its base requires about 4,000 men for the protection of each line of communication. With the help of figures such as these one may, perhaps, understand the more readily how it is that a Commander-in-Chief, of merciless disposition, and wanting to retain the active services of every soldier he possibly can in the interests of an early and successful advance will, by spreading a feeling of "terror" among the civil population, seek to reduce to as low a figure as circumstances will permit the number of men he must leave behind to guard his lines of rail communication. These considerations will be found to apply with the greater force when it is remembered that in the _Franco-Prussian War of 1870-71_ the Prussians had to adopt an especially elaborate system for safeguarding their lines of communication with Germany during the time they occupied French territory. At each railway station they placed a guard formed of detachments of the Landwehr, while small detachments were stationed in towns and villages in the neighbourhood. In each signal-box a detachment of troops was stationed, and the whole line of railway was patrolled from posts established along it at distances of every three or four miles. Altogether, the Germans are said to have employed, on over 2,000 miles of French railway lines controlled by them, as many as 100,000 troops for protective purposes only; and even then the _franc-tireurs_ were able to cause many interruptions. Under a Prussian regulation dated May 2, 1867, it was laid down that after the restoration of any lines taken possession of in an enemy's territory, notice should be given that in the event of any further damage being done to the railway, the locality would be subject to a fine of at least 500 thalers, the belongings of the inhabitants would be liable to seizure, and the local authorities might be arrested. As a further precautionary measure in the war of 1870-71, the Germans took a hint from the example of the Union Generals in the American Civil War by compelling a leading citizen of the district passed through to ride on the engine of each train run by them on French soil. In defence of this practice, the German General Staff say in their handbook on "The Usages of War"[9]:--