The facts above mentioned brought into my mind what is related of the plague at Smyrna and other places, that it disappears at the hottest part of the year. It is also curious and important to remark, that the true pestilence never has been heard of between the tropics. It is not easy to assign the cause of this effect of heat upon infection, as every thing relating to this subject is very obscure. We can conceive it to be owing to the greater degree of airiness which the heat of the climate makes necessary, or to the use of fewer woollen clothes. There may be something in the state of the body, particularly in the pores of the skin, which disposes them less to imbibe or produce the poisonous effluvia, or, when imbibed, it may more readily be thrown out by perspiration with the other acrimony of the blood; or more probably, as has been hinted above, the virulent matter is of such a degree of volatility as to be readily dissipated in a certain degree of heat[56]. There is a fact, which, though seemingly of a contrary tendency, yet is in reality in proof of the same opinion. It is, that these same diseases disappear in circumstances of great cold. When England was last visited by the plague, it disappeared in winter; and the same is observed at Moscow and other places. In this case the infectious matter is rendered _inert_, but not _extinct_, and the return of heat sets it afloat in the atmosphere, so as to expose it to human respiration. Dr. Guthrie informs us, that infection is entangled and fixed by the cold of winter on the doors and walls of the houses of the Russian peasants, and that upon the return of the warm season it is set loose by the thaw, and then becoming active, produces diseases. With regard to the West Indies, the precautions that have been laid down are chiefly necessary when a ship newly arrives in the climate; for it is during the first three or four months that sickness is apt to prevail. This does not depend upon any thing peculiar to the climate; for I have known ships arrive without being visited with any sickness. It seems to be owing, for the most part, to that flock of infection and disease imported from Europe exerting its effects, and when this has spent itself, the men remain in good health, unless exposed to the land air or other accidents; for the air at sea in those climates, as well as every where else, is extremely pure and wholesome, and there is no where that seamen are more healthy or comfortable. SECT. III. Of the FOUL AIR generated in a Ship. I mean here to distinguish the unwholesome vapour produced by the contents of the ship from the infection produced by the effluvia of men’s persons, which was treated of in the last section. The means of preventing this foul air from being generated are, cleanliness, dryness, and ventilation.