s, woods, or
such other impediments as serve to break the current in its progress
from the noxious source. It is an obvious fact, that the noxious cause,
or the exhalation in which it is enveloped, ascends as it traverses the
adjacent plain, and that its impression is augmented by the
adventitious force with which it strikes upon the subject of its
action.

"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 hea