2 
Existence and Position of 
[January, 
masses. (More than thirty years ago Prof. Kirkwood sup- 
posed that he had discovered the relation in the analogy 
which bears his name. The materials for testing and esta- 
blishing it were then, however, insufficient, and still remain 
so, leaving too many of the data uncertain and arbitrary). 
Could such relation be discovered, it could hardly fail to 
have a most important significance with respect to theories 
of the origin and development of the solar system.” . . . 
I do not know of the “ analogy,” but have seen in the 
“ Observatory ” for 1880 that a Mr. Kirkwood writes — “ If 
Laplace’s hypothesis cannot be sustained, we may conclude 
that each planet, at its origin, was separated from a very 
limited arc of the equatorial protuberance, or, in other 
words, that instead of the separation of a ring the centrifu- 
gal force produces a rupture at the point of least resistance 
in the equatorial belt.” Having in 1843* * * § given reasons for 
the insufficiency of the original form, pure and simple, of 
Laplace’s hypothesis and theory, I published in 1857!- a 
theory of the common origin, development, and present 
organisation of the solar system, and therefore feel interested 
in Mr. Kirkwood’s uncertain steps in the direction of a 
trodden path. 
This theory asserted the existence of two more great 
planets beyond Neptune, for which I got further evidence in 
1875. J Mr. D. P. Todd did therefore not stand alone with 
his meritorious inquiry into “ some evanescent perturbations 
in Neptune’s motions. § The peremptory assertions of Prof. 
George Forbes || are too cometary to be of account.^ 
It is most satisfactory that Prof. Young is not of the 
opinion of Prof. Newcomb. “ It is true that many inge- 
nious people employ themselves from time to time in working 
out numerical relations between the distances of the planets, 
their masses, their times of rotation, and so on, and will 
probably continue to do so, because the number of such 
relations which can be made to come somewhere near the 
exabt numbers is very great. This, however, does not indi- 
cate any law of Nature.” Does the accomplished astronomer 
* Weltgebaeude. Charlottenburg, 1843, bei Bauer ; and Frankfurt, 1879, 
bei Ruettgen. 
t Gedanken eines Nicht-Gelehrten. Philadelphia, 1857, bei Thomas, pages 
113 and following being translated as “Birth and Evolution of the Solar 
System ” in “ Journal of Science,” London, 1883, February to April, 
I Two Planets beyond Neptune, and the Own Motion of the Solar System. 
London, 1875, Wertheimer and Lea. 
§ Silliman’s American Journal of Science, 1881. 
|| Observatory. London, 1880, June. 
II ‘‘Astronomical Notes,” Journal of Science. London, 1881, November. 
