"It is thus that a position of three hundred paces from the margin of a swamp, on a level with the swamp itself, or but moderately elevated, is less unhealthy than one at six hundred on the same line of direction on an exposed height. The cause here strikes fully in its ascent; and as the atmosphere has a more varied temperature, and the succussions of the air are more irregular on the height than on the plain, the impression is more forcible, and the noxious effect more strongly marked. In accord with this principle, it is almost uniformly true, _coeteris paribus_, that diseases are more common, at least more violent, in broken, irregular, and hilly countries, where the temperature is liable to sudden changes, and where blasts descend with fury from the mountains, than in large and extensive inclined plains under the action of equal and gentle breezes only. "From this fact it becomes an object of the first consideration, in selecting ground for encampment, to guard against the impression of strong winds on their own account, independently of their proceeding from swamps, rivers, and noxious soils. "It is proved by experience, in armies as in civil life, that injury does not often result from simple wetting with rain when the person is fairly exposed in the open air, and habitually inured to the contingencies of weather. Irregular troops, which act in the advanced line of armies, and which have no other shelter from weather than a hedge or tree, rarely experience sickness--never, at least, the sickness which proceeds from contagion; hence it is inferred that the shelter of tents is not necessary for the preservation of health. Irregular troops, with contingent shelter only, are comparatively healthy, while sickness often rages with violence in the same scene, among those who have all the protection against the inclemencies of weather which can be furnished by canvas. The fact is verified by experience, and the cause of it is not of difficult explanation. When the earth is damp, the action of heat on its surface occasions the interior moisture to ascend. The heat of the bodies of a given number of men, confined within a tent of a given dimension, raises the temperature within the tent beyond the temperature of the common air outside the tent. The ascent of moisture is thus encouraged, generally by a change of temperature in the tent, and more particularly by the immediate or near contact of the heated bodies of the men with the surface of the earth. Moisture, as exhaled from the earth, is considered by observers of fact to be a cause which acts injuriously on health. Produced artificially by the accumulation of individuals in close tents, it may reasonably be supposed to produce its usual effects on armies. A cause of contagious influence, of fatal effect, is thus generated by accumulating soldiers in close and crowded tents, under the pretext of defending them from the inclemencies of the weather; and