THE Heat of the Air alone, where it is constant and uniform, does not appear to render Persons born in it, or long accustomed to it, any more unhealthful, than that which is more temperate. _Aristotle_[26] indeed says, a hot and dry _Southerly_ Wind will bring a _Pestilence_; but of such we have very few Instances, especially on this Part of the Globe. There is in _Livy_[27] an Account of a _Plague_ at _Rome_, from a great Drought; and _Nicephorus_[28] relates such another: But these generally come from some other manifest Causes besides Heat, and in Places not accustomed to a dry Air. BUT a hot and moist Air is very different. By _Moist_, is meant what arises from sudden or long Rains. This is the Constitution of Air that most Authors charge with being the greatest Instrument in _Pestilential Distempers_. _Hippocrates_[29] ascribes a great deal to such an Air, and relates a _Pestilence_ that had its Rise from great Heat, joined with _Southerly_ Winds and much Rain. _Galen_ is of the same Mind, as appears from several of his Writings[30], with many others too tedious to mention. The Truth of this is likewise manifest from the Histories of those Countries, where there are long settled Heats, and afterwards much Rain, as in several Parts of the _East-Indies_, which are known at such Times to be most grievously afflicted with Fevers and Diseases of a very malignant Kind. The same we are informed of from some Places in _Africa_,[31], _viz._ That if Showers fall soon upon the sultry Heats of _July_ and _August_, pestilential Distempers certainly ensue. It may be generally observed here too in our own Climate, that the most unhealthful Times are after warm Rains, and the more if the Air is then agitated but little with Winds. FROM a cold and moist Air, we have little complained of, as to their occasioning these Diseases, unless such a Constitution sets in immediately upon a contrary Extream; for all sudden Changes of Weather are more or less unhealthful, as well as in other Respects of living; for which Reason particularly, _Corn. Celsus_ advises to be very slow in all Alterations of Moment: And _Sanctorius_ frequently inculcates the same in his _Aphorisms_, and tells us[32] how it is hurtful, both to go suddenly out of a hot Air into a cold one, and out of a cold Air into a hot one; and is also very particular[33] in the Inconveniencies of such a Constitution of Air we are now speaking of, setting in after the Heat of Summer. _Hippocrates_[34] does tell us of a _Pestilence_ from long continued cold Rains, as likewise does _Fernelius_[35], but such Instances are not common.