EVOLUTION OF THE STELLAR SYSTEMS. Deep) 
the envelopes of the planets, and exhaustively examines the oscillations that will arise 
therein. Nor did he fail to consider fully the deviations from spherical form and the 
probable laws of density for the layers which compose the bodies of the planets. 
The effect of so monumental a work as the Mécanique Céleste was twofold: on the 
one hand it brought Physical Astronomy to an unexpected state of perfection, while on 
the other it produced the impression on the less creative minds that there were no great 
problems untouched by the master-mind of Laplace. His work had indeed well-nigh 
exhausted the theory of Celestial Mechanics, so far as it could be built upon the assumptions 
of rigid dynamics ; at least subsequent work has been for the most part little more than 
refinement or perfection of the methods and processes given in the Mécanique Céleste. The 
work of Laplace was designed for the solar system, and the idea that the universe is really 
composed of fluid bodies, self-luminous stars and nebulz in space, seems never to have 
occurred to him, or he would have foreseen that however adequate Rigid Dynamics may 
be for effecting a first approximation, the true theories of ultimate Celestial Mechanics 
must be founded upon the laws of viscous fluids in motion. So great is the influence of 
tradition that it is difficult for us to realize fully that the stars and nebule are viscous 
fluids, self-luminous liquid or gaseous masses, and that even in the solar system the 
bodies are all fluids of various viscosities. This new point of view respecting the actual 
facts of the universe has brought about an important modification in the nebular hypothesis 
and in the ultimate theories of Celestial Mechanics, of which we shall now give some 
account. 
About 1875, G. H. Darwin, who had qualified himself for the Law and been called to 
the Bar, on account of ill-health, abandoned his profession to undertake for Lord Kelvin 
some scientific work, which among other things included the reduction of a great mass of 
Indian tide observations with a view of throwing light upon the problem of the rigidity of 
the earth. This work, besides leading Lord Kelvin to the celebrated conclusion that the 
earth as a whole is “ probably more rigid than steel, but not quite so rigid as glass,” was 
the oceasion* of the younger Darwin developing the theory of bodily tides, or the 
theory of the tides which would arise in the earth on supposition that it is not rigid as at 
present, but a viscous fluid, as it must have been, according to Laplace, at some past age. 
While some allusions to bodily tides can be found in scientific literature as far back as 
Kant, and especially in the papers of Delaunay on the secular acceleration of the moon’s 
*In the Atlantic Monthly. for April, 1898, Prof. Darwin remarks: “It was very natural that Mr. See should 
find in certain tidal investigations which I undertook for Lord Kelvin the source of my papers, but as a fact the 
subject was brought before me in a somewhat different manner. Some unpublished experiments on the viscosity of 
pitch induced me to extend Lord Kelvin’s beautiful investigation of the strain of an elastic sphere to the tidal dis- 
tortion of a viscous planet. This naturally led to the consideration of tbe tides of an ocean lying on such a planet, 
which forms the subject of certain paragraphs now incorporated in Thomson and Tait’s Watwral Philosophy. 
A. P. S—VOL. xix. 2c, 
