HALLEY'S COMET. 



173 



stone has when whirled in a sling. Why, then, it will be asked, do not the 

 planets yield to this natural tendency ? What enables them to resist it ? To 

 this question no satisfactory answer can be given ; but the fact that the tendency 

 is resisted, being certain, the existence of some physical principle in which 

 the means of such resistance resides, is proved. As the tendency to fly off is 

 directed from the centre of the sun, the opposing physical influence must con- 

 sequently be directed toward that centre. This central influence is what has 

 been called gravitation. Although we are still ignorant of the nature or proxi- 

 mate cause of this force, and of its modus operandi, we have obtained a per- 

 fect knowledge of the laws by which it acts ; and this is all that is necessary 

 or material to enable us to follow out its consequences. By virtue of this force 

 of gravitation, then, the planetary masses receive a tendency to drop toward 

 the sun, which tendency equilibrates with the opposite tendency to fly away, 

 produced by their orbitual motion. On the exact equilibrium of these two op- 

 posite physical principles, depends the stability of the system. If the centrif- 

 ugal tendency proceeding from the orbitual motion were in excess, the planets 

 would fall off from the central body, and depart for ever into the depths of space ; ; 

 if, on the other hand, the central influence, or gravitation toward the sun, ex- < 

 isted in excess, these bodies would gradually approach that luminary, and finally 

 coalesce with his mass. 



Besides these bodies, the greater part of which have been long known, and 

 the motions of most of which have been in some degree understood, even from 

 remote antiquity, there is a still more numerous class of objects, whose appear- 

 ances in the system were of such a nature as to defy the powers of philosophi- 

 cal inquiry, until these powers received that prodigious accession of force 

 which was conferred upon them by the discoveries of Newton. Unlike planets, 

 comets do not present to us those individual characters above mentioned, by 

 which their identity may be determined. None of them have been satisfacto- 

 rily ascertained to be spherical bodies, nor indeed to have any definite shape. 

 Ic is certain that many of them possess no solid matter, but are masses con- 

 sisting entirely of aeriform or vaporous substances ; others are so surrounded 

 with this vaporous matter, that it is impossible, by any means of observation 

 which we possess, to discover whether this vapor enshrouds within it any solid 

 mass. The same vapor which thus envelopes the body (if such there be with- 

 in it), also conceals from us its features and individual character. Even the 

 limits of the vapor itself are subject to great change in each individual comet. 

 Within a few days they are sometimes observed to increase or diminish some 

 hundred fold. A comet appearing at distant intervals, presents, therefore, no 

 very obvious means of recognition. A like extent of surrounding vapor would 

 evidently be a fallible test of identity ; and not less inconclusive would it be to 

 infer diversity from a different extent of nebulosity. 



If a comet, like a planet, revolved round the sun in an orbit nearly circular, 

 it might be seen in every part of its path, and its identity might thus be estab- 

 lished independently of any peculiar characters in its appearance. But such 

 is not the course which comets are observed to take. These bodies usually 

 are observed to rush into our system suddenly and unexpectedly, from some 

 particular quarter of the universe. They first follow in a straight line, or nearly 

 so, the course by which they entered ; and this course is commonly directed 

 to some point not far removed from the sun. As they approach that luminary, 

 their path becomes curved; at first slightly, but afterward more and more ; the 

 curve being concave toward the sun. Having arrived at a certain least dis- 

 tance from the centre of our system, they again begin to recede from the sun, 

 and as their distance increases, their path becomes less and less curved ; until 

 at length they shoot off in a straight course, and make their exit from our sys- 



