This dreadful plague was preceded by comets and most violent earthquakes. A remarkable comet appeared in 536, supposed to be the great one observed by Sir Isaac Newton in 1680. This, we are told by astronomers, revolves round the sun in a period of 575 years; but the failure of astronomical predictions in the return of the expected comets of 1759 and 1789, shew the futility of such calculations. Another comet appeared in 539, and these comets were attended with an extraordinary paleness of the sun. Mr. Gibbon observes, that earthquakes, which he calls a fever of the earth, “raged with uncommon violence during the reign of Justinian. Each year is marked by the repetition of earthquakes of such character, that Constantinople has been shaken above forty days; of such extent, that the shock has been communicated to the whole surface of the globe, or at least of the Roman empire. An impulsive or vibrating motion was felt; enormous chasms were opened, huge and heavy bodies were discharged into the air, the sea alternately advanced and retreated beyond its ordinary bounds, and a mountain was torn from Liburnia, and thrown into the waves, where it protected as a mole the new harbour of Botrys in Phenicia.” According to Dr. Sydenham the plague at London in 1665 was preceded by a very cold winter; the first continued till spring and went off suddenly towards the end of March. Peripneumonies, pleurisies, quinsies, and other inflammatory disorders, then made their appearance, along with an epidemic fever of a particular kind, which did not yield to the remedies successful in other epidemics. About the middle of the year the plague began, and increased in violence till the autumnal equinox, when it began to abate, and by the ensuing spring was entirely gone. Our author says that the plague seldom rages violently in England but once in thirty or forty years; but since his time, which is upwards of a century, no plague hath appeared. He supposes the plague and other epidemics to depend on some secret constitution of the air, but pretends not to say what that constitution is. But, besides this constitution, he is of opinion that there must be another circumstance, viz. the receiving the effluvia or seminium from an infected person. Thus he supposes that a single infected person is sufficient to poison a whole country; the general mass of atmosphere being infected by the breath of the diseased and the effluvia of the dead bodies. “Thus (says he) the way of propagating this dreadful disease by infection is rendered entirely unnecessary; for though a person be most cautiously removed from the infected, yet the air received in by breathing will of itself be sufficient to infect him, provided his juices be disposed to receive the infection. I much doubt, if the disposition of the air, though it be pestilential, is of itself able to produce the plague; but the plague being always in some place or other, it is conveyed by