broken, or so completely destroyed that they can never be used again; the most that can be done with dykes is to cut them, which is not doing much. If there is a river in the middle, the bridge can of course be taken away, but the whole passage will not by that means be destroyed in the same degree as that of a large river by the destruction of a bridge. The natural consequence is that dykes which exist must always be occupied in force and strenuously defended if we desire to derive any general advantage from the morass.
On the one hand, therefore, we are compelled to adopt a local defence, and on the other, such a defence is favoured by the difficulty of passing at other parts. From these two peculiarities the result is, that the defence of a swamp must be more local and passive than that of a river.
It follows from this that we must be stronger in a relative degree than in the direct defence of a river, consequently that the line of defence must not be of great length, especially in cultivated countries, where the number of passages, even under the most favourable circumstances for defence, is still very great.
In this respect, therefore, swamps are inferior to great rivers, and this is a point of great importance, for all local defence is illusory and dangerous to an extreme. But if we reflect that such swamps and low grounds generally have a breadth with which that of the largest rivers in Europe bears no comparison, and that consequently a post stationed for the defence of a passage is never in danger of being overpowered by the fire from the other side, that the effects of its own fire over a long narrow dyke is greatly increased, and that the time required to pass such a defile, perhaps a quarter or half a mile long, is much longer than would suffice to pass an ordinary bridge: if we consider all this, we must admit that such low lands and morasses, if means of crossing are not too numerous, belong to the strongest lines of defence which can be formed.