370 OF THE PHÆNOMENA AND AFFECTIONS

tain stated times, like those of the planets.This indeed has been demonstrated by Dr. Hal-ley, who, from the theory of Newton, has cal-culated tables for determining the orbits of thecomets, which, in several instances, have beenfound agreeable to observation.

The comet which appeared in the year 1661,was seen before in the same orbit, and under thesame circumstances, in the year 1532, whichstiows its period to be 129 years; and if it re-appear in the year 1790, its identity will be stillfurther confirmed. So, also, the comet whichappeared in the years 1456, 1531, 1607, 1682,and 1759, is determined to revolve round thefun in a period of about 76 years. And thatremarkable comet which was observed in theyear 1680, is strewn to be the same with the onethat appeared in the year 1106, its period being

575 y ears -

The distance of this comet from the fun, whenin its perihelium, was to the distance of theearth from the fun, as 6 to 1000; its heat,therefore, at that time, was to the heat of thesummer’s fun with us, in the ratio of about28000 to 1 : so that, according to Newton, thesurface of this comet, supposing it to be com-posed of a matter like our earth, would haveacquired a heat, about 2000 times greater thanthat of red, hot iron; which is so intense, thatvapours, exhalations, and every volatile matter

must