unassailable; to give this requisite strength, notwithstanding the required extension, is not within the compass of the art of field fortification; it is therefore a fundamental condition that such a camp must derive part of its strength from natural impediments of ground which render many places impassable and others difficult to pass. In order, therefore, to be able to apply this defensive means, it is necessary to find such a spot, and when that is wanting, the object cannot be attained merely by field works. These considerations relate more immediately to tactical results in order that we may first establish the existence of this strategic means; we mention as examples for illustration, Pirna, Bunzelwitz, Colberg, Torres Vedras, and Drissa. Now, as respects the strategic properties and effects. The first condition is naturally that the force which occupies this camp shall have its subsistence secured for some time, that is, for as long as we think the camp will be required, and this is only possible when the position has behind it a port, like Colberg and Torres Vedras, or stands in connection with a fortress like Bunzelwitz and Pirna, or has large depôts within itself or in the immediate vicinity, like Drissa.
It is only in the first case that the provisioning can be ensured for any time we please; in the second and third cases, it can only be so for a more or less limited time, so that in this point there is always danger. From this appears how the difficulty of subsistence debars the use of many strong points which otherwise would be suitable for entrenched positions, and, therefore, makes those that are eligible scarce.
In order to ascertain the eligibility of a position of this description, its advantages and defects, we must ask ourselves what the aggressor can do against it.
a. The assailant can pass by this strong position, pursue his enterprise, and watch the position with a greater or less force.