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.
An indirect defence, such as we made ourselves acquainted with in the case of streams and rivers, in which obstacles of ground are made use of to bring on a great battle under advantageous circumstances, is generally quite as applicable to morasses.
The third method of a river-defence by means of a position on the enemy’s side would be too hazardous on account of the toilsome nature of the crossing.
It is extremely dangerous to venture on the defence of such morasses, soft meadows, bogs, etc., as are not quite impassable beyond the dykes. One single line of crossing discovered by the enemy is sufficient to pierce the whole line of defence which, in case of a serious resistance, is always attended with great loss to the defender.
B. Inundations
Now we have still to consider inundations. As defensive means and also as phenomena in the natural world they have unquestionably the nearest resemblance to morasses.
They are not common certainly; perhaps Holland is the only country in Europe where they constitute a phenomenon which makes them worth notice in connection with our object; but just that country, on account of the remarkable campaigns of 1672 and 1787, as well as on account of its important relation in itself to both France and Germany, obliges us to devote some consideration to this matter.
The character of these Dutch inundations differs from ordinary swampy and impassable wet low lands in the following respects:
The soil itself is dry and consists either of dry meadows or of cultivated fields.
For purposes of irrigation or of drainage, a number of small ditches of greater or loss depth and breadth intersect the country in such a way that they may be seen running in lines in parallel directions.
Larger canals, inclosed by dykes and intended for irrigation, drainage, and transit of vessels, run through the country in all possible directions and are of such a size that they can only be passed on bridges.
The level of the ground throughout the whole district subject to inundation, lies perceptibly under the level of the sea, therefore, of course, under that of the canals.