344 ANNUAL OF SCIENTIFIC DISCOVERY. 



If any form of the nebular hypothesis was to be adopted, it was 

 necessary to consider their origin. Upon that theory, the readiest 

 way to account for their existence would be to suppose them stran- 

 gers to all systems, being produced from portions of the nebulous 

 fluid left between the spheres of stellar attraction. His own opinion 

 was, however, that they are component parts of our system, and that 

 the comets within every system belong to that system. 



There were two classes "of arguments which might be produced. 

 The first arising from the nature of their orbits. from their not be- 

 ing hyperbolical. Of the hundred comets which had been carefully 

 observed, and whose orbits had been accurately computed during the 

 last century, not one had been shown to have a decidedly hyperbolical 

 orbit. But if the comets do not belong to our system, one half of 

 them, upon the average, ought to move in orbits decidedly hyperboli- 

 cal. He came to this conclusion upon the ground that our system 

 is moving in space. The very point towards which we are mov- 

 ing had been determined, and very recently, in a paper upon stellar 

 astronomy, by Struve, the deduction had been given of the very 

 amount of the motion of this system in space. Its velocity was com- 

 puted as about one fourth of the velocity of the earth's motion in its 

 orbit. It would amount to the' same thing, to suppose the solar sys- 

 tem to be placed in a stream of stars to which the comets belong, and 

 the average velocity of which the comets would possess. Now, by 

 the laws of motion, if the comet came into the solar system with no 

 velocity at all, its orbit would be a parabola; but if its velocity was 

 sensible, it would move in a hyperbola, the form of which would be 

 exactly dependent upon the amount of this velocity. It had been 

 shown by Laplace, that the direction does not influence the character 

 of the form of the orbit, but that from the velocity alone, at a given 

 distance from the sun, it can be determined. But if the comets did 

 not belong to the solar system, there ought to be some of them with 

 very remarkable hyperbolical eccentricity; so that the fact that there 

 are no comets with hyperbolic orbits seemed to be in itself almost de- 

 cisive proof that they do belong to the solar system. Another effect 

 of this motion in space would be, that the comets would more fre- 

 quently enter the system upon that side towards which we are moving; 

 which was not found to be the case. 



Prof. Peirce then showed, that, according to the doctrine of chances 

 by Laplace, the chance was 71 to 1 that there was an actual 

 law regulating the distribution of comets, and that they really be- 

 long to the solar system. A question would then occur, How are 

 the comets connected with the nebular hypothesis? If a comet had 

 been sent out from the sun by expansion, when it came back it could 

 hardly escape from falling again into the sun, even if thrown tangen- 

 tially. If sent off from the planets, and Lagrange had found the 

 necessary force to be comparatively small, the inclination and direc- 

 tion of the orbits would be about what they actually are ; with a ten- 

 dency towards the plane of the ecliptic, and towards direct motion. 

 The great difficulty was in making the force exactly sufficient to pro= 

 duce the parabola, or lengthened ellipse ; for if it was more than this. 



