394 
all other objections got over, there is one which is very obvions 
as regards the motion of our own earth round the sun as a 
planet in a resisting medium (whether it applies to the other 
planets or not) ; and it is an objection which, so far as I am 
aware, is not only not obviated nor answered in any book on 
astronomy, but it is one which, like most if not all of those I 
have now adduced, appears never to have been once considered 
by astronomers, but, on the contrary, is simply and altogether 
ignored. And it is this : — That however easy it might be, 
apart from dynamical considerations, to accept the Newtonian 
theory of the earth and planets revolving round the sun in 
spaces void of resistance, it is impossible not to perceive insu- 
perable difficulties to their doing so in a resisting medium, if, 
like the earth, they all have atmospheres. Getting rid of the 
vortices of Des Cartes, in and by which the planets were sup- 
posed to be carried round the sun in their orbits, indeed, 
necessitated a free space for these bodies to move in ; for, 
however easy it may be to conceive that solid bodies might 
move with immense velocity through thin air or ether, and 
yet retain their form, this cannot be rationally imagined of 
bodies having circumambient atmospheres like the earth; for 
in a resisting medium the earth, with its air, would soon 
assume the form of a comet, and in revolving in its orbit 
would carry all its light atmosphere and floating vapours 
behind it. And so of all the planets, if they have vaporous 
atmospheres. 
28. But the other corollary referred to (§ 13), is replete 
with still graver difficulties. It was, no doubt, introduced by 
Newton, who, as I have already said, knew nothing of solar 
motion in space, with the view of explaining or accounting for 
the motions of the satellites round their primary planets on 
his theory. The analogy between the motions of the sun and 
planets if the sun moved onwards in space, and the motions of 
any planet and its satellites round the sun, is perfect. In a 
letter which appeared in the Astronomical Register for Feb- 
ruary, 1864,* on “ The Motion of the Solar System in Space,” 
I wrote as follows : — 
“ Again, if the sun moves in space, the variation in the orbital velocities of 
all the bodies that revolve round it must differ, by the whole amount of the 
sun’s motion, when at right angles to its path, twice every revolution they 
make. For instance, taking the sun’s motion as 18,000 miles an hour, [the 
rate supposed by MM. Argelander, Struve, and Peters,] the earth’s mean 
orbital velocity of 65,000 miles an hour must sometimes be 65,000-}- 18,000= 
* London : J. D. Potter, Poultry, and King Street, Tower Hill. 
