The President's Address. 281 



fragment had proved its existence. This law we owe to Mr 

 Daniel Kirkwood of Pottsville, a humble American,* who, 

 like the illustrious Kepler, struggled to find something new 

 among the arithmetical relations of the planetary elements. 

 Between every two adjacent planets there is a point where 

 their attractions are equal. If we call the distance of this 

 point from the Sun the radius of a planet's sphere of attrac- 

 tion, then Mr Kirk wood's law is, that in every planet the 

 square of the length of its year, reckoned in days, varies as 

 the cube of the radius of its sphere of attraction. This law 

 has been verified by more than one American astronomer, 

 and there can be no doubt, as one of them expresses it, that 

 it is at least a physical fact in the mechanism of our system. 

 This law requires the existence of a planet between Mars 

 and Jupiter, and it follows from the law that the broken 

 planet must have been a little larger than Mars, or about 

 6000 miles in diameter, and that the length of its day must 

 have been about 67J hours. The American astronomers re- 

 gard this law as amounting to a demonstration of the nebular 

 hypothesis of Laplace ; but we venture to say that this 

 opinion will not be adopted by the astronomers of England. 

 Among the more recent discoveries within the bounds of our 

 own system, I cannot omit to mention those of our distin- 

 guished countryman, Mr Lassell of Liverpool. By means of a 

 fine 20-feet reflector, constructed by himself, he detected the 

 satellite of Neptune, and more recently an eighth satellite 

 circulating round Saturn — a discovery which was made on 

 the very same day, by Mr Bond, Director of the Observatory 

 of Cambridge, in the United States. Mr Lassell has still 

 more recently, and under a singularly favourable state of the 

 atmosphere, observed the very minute, but extremely black, 

 shadow of the ring of Saturn upon the body of the planet. 

 He observed the line of shadow to be notched, as it were, and 

 almost broken up into a line of dots — thus indicating moun- 

 tains upon the plane of the ring — mountains, doubtless, raised 

 by the same internal forces, and answering the same ends, 

 as those of our own globe. In passing from our solar sys- 



* For account of Kirkwood*s analogy in the Periods of Rotation of the 

 Primary Planets, vidt Edinburgh New Philosophical Journal, vol. xlix. p. 165. 



