4. The summer and autumnal diseases of our country have often followed the erection of mill-dams. They may easily be obviated by surrounding those receptacles of water with trees, which prevent the sun's acting upon their shores, so as to exhale miasmata from them. Trees planted upon the sides of creeks and rivers, near a house, serve the same salutary purpose. 5. It has often been observed, that families enjoy good health, for many years, in the swamps of Delaware and North-Carolina, while they are in their natural state, but that sickness always follows the action of the rays of the sun upon the moist surface of the earth, after they are cleared. For this reason, the cultivation of a country should always follow the cutting down of its timber, in order to prevent the new ground becoming, by its exhalations, a source of disease. 6. In commercial cities, no vessel that arrives with a cargo of putrescent articles should ever be suffered to approach a wharf, before the air that has been confined in her hold has been discharged. The same thing should be done after the arrival of a vessel from a distant or hot country, though her cargo be not capable of putrefaction, for air acquires a morbid quality by stagnating contiguous to wood, under circumstances formerly mentioned. All these modes of removing the causes of malignant and yellow fevers, and of promoting strict and universal cleanliness, are of more consequence in the middle and northern states of America, than in countries uniformly warm, inasmuch as the disease may be taken as often as our inhabitants are exposed to its sources. In the West-Indies, a second attack of the yellow fever is prevented by the insensibility induced upon the system, by its being constantly exposed to the impressions of heat and exhalation. After a seasoning, as it is called, or a residence of two or three years in those islands, the miasmata affect the old settlers, as they do the natives, only with mild remittents. Nearly the same thing takes place at Madras, in the East-Indies, where, Dr. Clark says, the exhalations which bring on bilious fevers, colic, cholera, and spasmodic affections in new comers, produce a puking in the morning, only in old residents. But very different is the condition of the inhabitants of the middle and northern states of America, in whom the winters prevent the acquisition of habits of insensibility to the heat and exhalations of the previous summers, and thus place them every year in the condition of new comers in the West and East-Indies, or of persons who have spent two or three years in a cold climate. This circumstance increases the danger of depopulation from our malignant epidemics, and should produce corresponding exertions to prevent them.