The air is so evidently connected with human life, that it has been from the earliest ages accounted the source of pestilential diseases, though, as none of the more obvious qualities of it, such as heat, cold, moisture, or dryness, appeared to be connected with them, they were generally supposed to proceed from the action of some unknown natural cause, or from that of the Deity himself. Some, however, have also been of opinion that plagues might originate from the obvious qualities of the air in conjunction with certain effluvia from putrid vegetable or animal bodies. Thus, in several plagues mentioned in ancient history, we find swarms of dead locusts, grasshoppers, the carcases of those slain in battle, crowded houses, and filth of all kinds, assigned as causes. This opinion was adopted by Dr. Mead, and he gives the following account of the origin of the plague in Egypt. “Grand Cairo is crowded with inhabitants, who for the most part live very poorly and nastily; the streets are narrow and close; it is situate in a sandy plain, at the foot of a mountain, which, by keeping off the winds that would refresh the air, makes the heats very stifling. Through the midst of the city passes a great canal, which is filled with water at the overflowing of the Nile; and, after the river decreases, is gradually dried up: into this canal the people throw all manner of carrion, filth, &c. so that the stench which arises from this and the mud together is insufferably offensive. In this posture of things, the plague every year preys upon the inhabitants, and is only stopped when the Nile, by overflowing, washes away this load of filth; the cold winds, which set in at the same time, lending their assistance by purifying the air.” He then proceeds to account for the plagues in Ethiopia in the manner above related, viz. by the prodigious swarms of locusts, which sometimes occasion a famine by devouring the fruits of the earth, and, when they happen to be cast by the winds into the sea, occasion a pestilence; the putrefaction being heightened by the intemperance of the climate, which here is so great that it is infested with violent rains for three or four months together; and it is particularly observed of this country, that the plague usually invades it whenever rains fall during the sultry heats of July and August. He next takes notice of what the Arabians say of the origin of the plague in Ethiopia, viz. that it is brought on by unseasonable moistures, heats, and want of winds. But, whatever truth may be in the account given of the Ethiopic plagues, the testimonies already produced in this treatise are sufficient to render it very doubtful, at least, whether the plague ever does originate in Cairo, or any other place in Egypt. Besides, if we once admit the existence of any thing as a cause adequate to the production of a certain effect, wherever that cause exists the effect ought certainly to follow, unless where we