I. Excessive and continued heat is, perhaps, the most fruitful source of cholera. Thus we find, that the disease makes its first appearance in the commencement of the hot weather, increases and becomes more fatal with the rise of the thermometer, and declines with the return of cool weather in autumn. During its continuance, it may be observed to vary with every permanent change of temperature. A few very hot days in succession, in the 6th month, are sufficient to call it into action; and during the height of its prevalence, a spell of cold weather will diminish, if not suppress it. In the summer of 1806, which was remarkably cool and pleasant, there was very little of the disease; and generally in moderate summers, it is much less prevalent than in those of a contrary character. I believe that it is by a direct operation on the system, and not by the generation of miasmata, that heat proves so deleterious to the infant. In the country, where miasmata are most abundant, there is comparatively little cholera; for the heat of the sun is there moderated by the free circulation of the air; and the debilitating operation of the high temperature of the day is counteracted by the refreshing coolness of the morning and evening. It is in the close air of cities, that the complaint flourishes with greatest vigour; and the most confined situations are the most favourable to its production. Let any one take a walk, in a summer's morning, through the thickly built lanes and alleys of Philadelphia. He will be struck with the appearance of the children, reclining their heads, as if exhausted, upon the breast of their mothers, with a pale and languid countenance, a cool and clammy skin, a shrunk neck, and other signs of debility, arising from their confinement, during the night, to close and hot apartments. It will readily be believed, that such places are the very hot beds of cholera.