ON THE SOLAR SYSTEM. 513 



suffered by the inhabitants of the planet might be merely temporary and lo- 

 cal: the chances are, however, much greater, that a comet might interfere in 

 such a manner with a planet, as to deflect it a little from its course, and retire 

 again without coming actually into contact with it. 



Nearly 500 comets are recorded to have been seen at different times, and 

 the orbits of about a hundred have been correctly ascertained : but ue have 

 no opportunity of observing a sufficient portion of the orbit of any comet, to 

 determine with accuracy the whole of its form as an ellipsis, since the part 

 which is within the limits of our observation does not sensibly differ from the 

 parabola, which would be the result of an ellipsis prolonged without end. 



Two comets at least, or perhaps three, have been recognised in their re- 

 turn. A comet appeared in 1770, which Prosperin suspected to move in an 

 orbit materially different from a parabola: Mr. Lexell determined its period 

 to be 5 years and 7 months, and its extreme distances to be between the 

 orbits of Jupiter and of Mercury; but it does not appear that any sub- 

 sequent observations have confirmed his theory. It has, however, been cal- 

 culated, that supposing the theory correct, it must afterwards have approach- 

 ed so near to Jupiter as to have the form of its orbit entirely changed. 



Dr. Halley foretold the return of a comet about 1758, which had appeared 

 in 1531, in 1607. and in 1682, at intervals of about 75 years; and with 

 Clairaut's further correction for the perturbations of Jupiter and Saturn, the 

 time agreed within about a month. The mean distance of this comet from 

 the sun must be less than that of the Georgian planet; so that by improving 

 our telescopes still more highly, we may, perhaps, hereafter be able to con- 

 vert some of the comets into planets, so far as their remaining always visible 

 would entitle them to the appellation. Dr. Halley also supposed the comet 

 of 1680 to have been seen in 1 106, in 531, and in the year 44 before Christ, 

 having a period of 575 years; and it has been suspected that the comets of 

 1556 and 1264 were the same, the interval being 292 years ; a conjecture which 

 will either be confirmed or confuted in the year 1 848. Some persons have 

 even doubted of the perfect coincidence of the orbits of any comets, seen at 

 different times, with each other, and have been disposed to consider them as 



VOL. I. 3 u 



