as to which they can do no more than prophesy that they will return to our parts of Space some day. These different circumstances bring it about that comets vary greatly in the distances to which they recede from the Sun. Whilst the orbit of the Comet known as Encke's is contained within the orbit of Jupiter, the orbit of Halley's Comet stretches out beyond that of Neptune, whilst many other comets recede to far greater distances than this. A comet can only come back to the Sun after having appeared and then disappeared, provided it moves in an elliptic orbit. The chance visitors spoken of in a previous paragraph pursue curved paths known as "parabolas" or "hyperbolas"; but the further consideration of these details is reserved for a later chapter.
The density and also the mass of comets is exceedingly small, and their tails consist of matter of such extreme tenuity that even small stars are visible through them, a fact first recorded by Seneca. That the matter of comets, whatever it may be, is exceedingly rare is sufficiently proved by the fact that instances are on record of comets having passed very near to some of the planets without disturbing in any appreciable degree the motions of the said planets. For instance, the Comet of 1770 (Lexell's) in its approach towards the Sun enveloped the satellites of Jupiter, and remained near them for 4 months without affecting them as far as we know. From this fact it can be shown that the mass of this comet could not have been so much as 15000 that of the Earth. This comet came very near to the Earth on July 1, 1770: its distance at 5h on that day being about 112 millions of miles. Had its matter been equal in quantity to that of the Earth its attractive force would have caused the Earth to move in an orbit so much larger than it does at present that the length of the year would have been increased by 2h 47m, yet no sensible change took place.
The idea of any danger happening to our planet, or to any other planet, from the advent of any of these wandering strangers, may be dismissed once and for all, especially as we now know that the Earth passed bodily through the tail of the great Comet of 1861, on June 30 of that year.
