XXXVI Mr. Pickering’s Eulogy on 
in part depend upon the action of the planets, are then discussed 
in order, and a comparison made of the theory with observation ; 
to which the author adds the very interesting and delicate investi- 
gation of the secular variations, in the motions of the Moon and 
Earth, which may be produced by the resistance of an ethereal 
fluid surrounding the Sun; the existence of such a fluid being 
assumed by La Place as possible. Dr. Bowditch adds, in a note, 
that the existence of such a resisting medium is now considered 
highly probable, in consequence of the observed decrease of the 
times of revolution of Encke’s comet, in its successive appearances 
between the years 1786 and 1829; and he further observes, that 
the extreme rarity of the mass of this comet, makes it peculiarly 
well adapted to the discovery of the effect of such a resisting ethe- 
real fluid, which cannot, however, produce any sensible effect on 
the large and dense bodies of the planets and satellites.* 
In the eighth book, with which the fourth volume opens, La Place 
considers the perturbations of the Satellites of Jupiter, Saturn, and 
Uranus ; the masses of the satellites, the oblateness of the planet 
Jupiter, and the eccentricities and inclinations of the orbits of the 
satellites. The phenomena of the Satellites are of great importance ; 
for, as La Place observes, although they have been discovered only 
two centuries, and their eclipses have been observed but a century 
and a half, yet, during “this short interval, these bodies have pre- 
sented to our view, by the rapidity of their revolutions, all those 
great changes which time produces with extreme slowness in the 
planetary orbits; the system of the satellites being an image of 
that of the planets.” + 
He next gives the theory of Comets; their perturbations at a 
* Bowditch’s La Place, Vol. III. p. 678. 
+ Mécanique Céleste, Vol. IV. Preface. 
