must not overlook the endless number of other cases in which they were respected, even when there existed in the place the most urgent need for relief; and when the commander on the defensive side was a man of great enterprise, as in 1708, when Villars did not venture to attack the allies in their lines at Lille. Frederick the Great at Olmütz, 1758, and at Dresden, 1760, although he had no regular lines of circumvallation, had a system which in all essentials was identical; he used the same army to carry on the siege, and also as a covering army. The distance of the Austrian army induced him to adopt this plan at Olmütz, but the loss of his convoy at Domstädtel made him repent it; at Dresden in 1760 the motives which led him to this mode of proceeding, were his contempt for the German States’ imperial army, and his desire to take Dresden as soon as possible.
Lastly, it is a disadvantage in lines of circumvallation, that in case of a reverse it is more difficult to save the siege train. If a defeat is sustained at a distance of one or more days’ march from the place besieged, the siege may be raised before the enemy can arrive, and the heavy trains may, in the mean time, gain also a day’s march.
In taking up a position for an army of observation, an important question to be considered is the distance at which it should be placed from the besieged place. This question will, in most cases, be decided by the nature of the country, or by the position of other armies or corps with which the besiegers have to remain in communication. In other respects, it is easy to see that, with a greater distance, the siege is better covered, but that by a smaller distance, not exceeding a few miles, the two armies are better able to afford each other mutual support.