Kirkwood^s Analog^/. 165 



such, for instance, as the Echinoid and Asteroid Crinoids of the for- 

 mer ages. 



The fact that these different types may thus present complications 

 of their character, or appear more or less pure and typical, goes 

 further to shew how deeply diversified the plan of creation is, and 

 how many relations should be simultaneously understood before we 

 are prepared to have a full insight into the plan of creation. There 

 we see one type forming simply, and alone, the first link of a progres- 

 sive series. There wo see another which foreshadows types, which 

 appear isolate afterwards. There we see a third, which, in its full 

 development, exemplifies a state which is transient only in higher 

 representatives of the same family. And then, again, we see these 

 different relations running into each other, and reminding us that, 

 however difficult it may be for us to see at one glance all this diversity 

 of relations, there is, notwithstanding, an intelligence which not only 

 conceived these various combinations, but called them into real exist- 

 ence in a long succession of ages. — L. Agassiz in the American 

 Association for the Advancement of Science, August 1849. 



On a new Analogy in the Periods of Rotation of the Primary 

 Planets discovered by Daniel Kirkwood of Pottsville, Penn- 

 sylvania. 



At the recent meeting of the Association for the Advancement of 

 Science, an announcement was made, which, if it is found to be cor- 

 rect, will be regarded as relating to one of the most important dis- 

 coveries which have been made in astronomy for years. It is no less 

 than a new law of the solar system, closely resembling those of Kep- 

 ler, which form the groundwork of many of the problems of astronomy. 

 Mr S. C. Walker read to the Association a letter from Mr Daniel 

 Kirkwood, of Pottsville, Pa., the discoverer of this new law, from 

 which we make some extracts, omitting all that refers to the higher 

 branches of mathematics. 



" While we have in the law of Kepler a bond of mutual relation- 

 ship between the planets, as regards their revolutions around the 

 sun, it is remarkable that no law regulating their rotations on their 

 axes has ever been discovered. For several years I have had little 

 doubt of the existence of such a law in nature, and have been en- 

 gaged, as circumstances would permit, in attempting its development. 

 I have at length arrived at results, which, if they do not justify me 

 in announcing the solution of this important and interesting problem, 

 must at least be regarded as astonishing coincidences." 



After stating some equations, he gives the following tables as the 

 data on which he has proceeded : — 



