LET. XXI. OF THE SUN, MOON, AND PLANETS. 369

able. When they are near the sun, their mo-tion is very rapid, and in the more distant partsof their orbits extremely stow; so that their vi-cissitudes, in this respect, are as much in theextreme, as what they undergo from heat andcold.

When a comet arrives within a certain dis-tance of the sun, it emits a fume or vapour calledits tail; which stiows that they contain a por-tion of matter considerably more rare and vola-tile than any on the earth; for the tail begins toappear when they are yet in a higher, and con-sequently a colder region, than Mars. In everysituation of the comet the tail is always directedto that part of the heavens which is nearly op-posite to the fun; and is always greater after thecomet has passed its perihelium, than during itsapproach towards it, being greatest of all whenit has just left that point.

That part of a comet’s orbit, which comes un-der our inspection, is To small in proportion tothe whole, that it differs but little from a para-bola, for which reason the dimensions of theirorbits and periodical times cannot be ascertained,with any degree of precision, from a single ob-servation. But from the re-appearance of severalcomets, after long intervals of time, in the sameregion of the heavens; and from their beingfound to move in the same curve, it is evidentthat their revolutions must be performed in cer-B b tain