"It is consonant with the experience of military people, in all ages and in all countries, that camp diseases most abound near the muddy banks of large rivers, near swamps and ponds, and on grounds which have been recently stripped of their woods. The fact is precise, but it has been set aside to make way for an opinion. It was assumed, about half a century since, by a celebrated army physician, that camp diseases originated from causes of putrefaction, and that putrefaction is connected radically with a stagnant condition of the air. "As streams of air usually proceed along rivers with more certainty and force than in other places, and as there is evidently a more certain movement of air, that is, more wind on open grounds than among woods and thickets, this sole consideration, without any regard to experience, influenced opinion, gave currency to the destructive maxim that the banks of rivers, open grounds, and exposed heights are the most eligible situations for the encampment of troops. They are the best ventilated; they must, if the theory be true, be the most healthy. "The fact is the reverse; but, demonstrative as the fact may be, fashion has more influence than multiplied examples of fact experimentally proved. Encampments are still formed in the vicinity of swamps, or on grounds which are newly cleared of their woods, in obedience to theory, and contrary to fact. "It is prudent, as now said, in _selecting ground for encampment_, to avoid the immediate vicinity of swamps and rivers. The air is there noxious; but, as its influence thence originating does not extend beyond a certain limit, it is a matter of some importance to ascertain to what distance it does extend; because, if circumstances do not permit that the encampment be removed out of its reach, prudence directs that remedies be applied to weaken the force of its pernicious impressions. "The remedies consist in the interposition of rising grounds, 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.