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.» t portion as they approach him, which shews*that she tail depends upon the degree of heat whichthe Cj-.net receives from the Sun. Secondly, thetails of Comets are always opposite the San, in thefame manner as steam or smoke ascends from thefire that produced it. Thirdly, the tails of Cometsperfectly resemble the smoke or vapor of a burn-ing coal, for if the coal remains fixed, the smokeascends perpendicularly, but if the coal be in mo-tion it ascends obliquely, and the different appear-ances of the heads of Comets, greatly confirm thisopinion of their tails ; for smoke raised by a strongheat is blacker and grosser than when raised bya less; and accordingly the heads of Comets, hav-ing been more heated by the Sun, are less shiningthan when they were less heated, just as if theywere then obscured by such gross smoke.

Fourthly, these tails or vapors are likewisefound to participate of another property of ascend-ing vapor, which is, that when it ascends withthe greatest velocity, it is least incurvated.

And lastly, from the very near approach to theSun of the Comet of 1680, Sir Isaac Newton drawsanother Speculation ; to wit, that some people haveheretofore suggested, tho’ it be very difficult toprove, that the Sun, by a constant emission of lightand heat, does really diminish ; now should thatbe the cafe, yet might a Comet approach so nearits surface as to be attracted thereto, and therebysupply any decrease it may have hitherto under-gone.

The reader will easily perceive, that enteringfarther into this subject would have engaged me insuch sort of Calculations, as depend on principlesthe generality of people are unacquainted with,

and