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22 _ RESULTS OF RECENT RESEARCHES ON THE 
observations, must be ranked as incomparably the most profound yet attained in any 
branch of Physical Science. 
Notwithstanding these splendid triumphs of the science of Celestial Mechanics, an 
even greater and more recondite work remains to be done in a closely related field. This 
is the investigation of the origin and cosmical history of the planetary and other systems 
observed in the immensity of space. Even if some credit for pioneer work on this 
problem be assigned to Kant, or, more remote still, to the Greeks of the pre-Socratic age, 
it yet remains true that Laplace is the real discoverer to whom we are indebted for the 
first ideas which proved fruitful for the advancement of science. About a century ago 
this great geometer outlined for the solar system the celebrated Nebular Hypothesis, upon 
which nearly all subsequent investigation has been based, and which has since been sub- 
stantially confirmed, though but very little modified until within the last twenty-five 
years. Passing over as irrelative in the present discussion the early work of Herschel 
and Rosse, Helmholtz and Kelyin, Newcomb and Lane, we come down to the modifica- 
tions introduced by Darwin about 1880. 
In establishing the theory of gravitation, Newton assigned also the true cause of the 
tides of the seas, though his explanation carried with it all the defects of the equilibrium 
theory. More than a century passed before the dynamical character of the problem of 
the oceanic tidal oscillations was clearly perceived, when Laplace developed and applied 
the true theory with all the penetration characteristic of that great mathematician. 
Yet in spite of the profundity which marks his treatment of the tides of the oceans, it 
seems never to have occurred to him, or at least he made no record of the fact, that the 
attraction of the moon necessarily produces tides in the body of, as well as in the aqueous 
layers covering, the earth. We need not be surprised at this omission on the part of 
Laplace and those who followed him, if we recall that for many years after the perfection 
of Analytical Mechanics by D’Alembert and Lagrange, the subject was treated wholly 
from the point of view of material particles, and the resulting system was what is now 
called Rigid Dynamics. Little attention was bestowed upon the theory of fluid motion, 
partly because of its intricacy, and. partly because there were no obvious applications of 
the results except in the case of the tides, already treated by Laplace with great penetra- 
tration and extreme generality. As mathematicians since the time of Newton had been 
occupied chiefly with the development of the theory of planetary perturbations along the 
line of rigid dynamics, it did not occur to them that they were building on a false 
premise, that in reality the heavenly bodies so far as known are not solid, but fluid, 
though Laplace with his usual sagacity had long foreseen that in the case of our planets 
the nuclei are covered with fluid layers held in equilibrium by the pressure and attraction 
of their parts. His grand treatment in the Mécanique Céleste recognizes the fluidity of 
