Thus hard Frost, strong cold and Northerly Winds, are found frequently to put an End to, or at least bridle the Fury of Contagious Diseases, and render them more mild and curable, as was observable in the Beginning of the last great Plague in _London_[m], and frequently taken Notice of in other places by the Writers on this Subject. Consonant to this we find in _Ægypt_, that the Rising of the _Nile_ by giving a fresh Motion to, and altering the Disposition of their stagnating and putrid Air, by the mild Vapours and Nitrous Exhalations[n] issuing from it, immediately checks the Raging of the Plague, and reduces it to a Fever of a more mild and curable Nature; insomuch that as _Purchas_ and others inform us, if there die in _Grand Caire_ 500 Persons of the Plague the Day before, yet upon the Increase of the River it ceases to be Pestilential, and none die of it[o]. And indeed it can hardly be imagin'd, how the Plague when it has once got establish'd in any Place, shou'd cease but with the Destruction of all or most of the Inhabitants, was it not checked by some Alteration in the Disposition of the Air, and gradually reduced to a Fever of a more mild and curable Disposition. [m] _Hodges_ de Peste. [n] _Boyle_'s determ. Nat. Effluv. cap. 4. _Plot_'s Nar. Hist. of _Staffordsh._ cap. 2. pag. 42. [o] _Purchas_ Pilgrim, lib. 6, cap. 7. _Sandy_'s Travels, lib. 2. cap. 97. It will I think be needless to show, that the Distempers here treated on are propagated by Contagion; But it may not be altogether unnecessary to explain by what Methods these Alterations in the Animal Oeconomy are brought about, and especially as the Means by which they are chiefly communicated, have not that I know of been fully examin'd. [Decoration] CHAP. IV. [Sidenote: _Of a Pestilence by Contagion._]