386 
'osition of solar motion, being precisely and necessarily in a 
•ycloid curve, like that described by a nail in the rim of a. 
wheel as it rolls along the ground ! And yet if solar motion 
n space be true, the earth and planets must all move in curves 
nore or less cycloidal, and all of them always with velocities 'plus 
md minus that of the velocity of the sun in the course of each 
.’evolution they make. Such is the incontrovertible result of 
some of the teachings of current physical astronomy, as it is 
now to be found in all our orthodox books on astronomy, ever 
since the first Herschel's time. But it is what rational men 
will be unable to believe, whenever they come to think. It is 
utterly inconsistent with all Newton's “ demonstrations," such 
as they are; and yet it has its foundation on the illogical 
so-called 6th Corollary " to the first proposition of the 
Principia. 
16. Following the second proposition of the Principia, and 
the scholium thereon, we have another somewhat extraordinary 
corollary. It contains one of the rare allusions to be found in 
the Principia to the possible existence of a resisting medium 
in space as affecting the motions of the heavenly bodies ; * and 
is as follows : — 
“ Cor. 2. And, even in resisting mediums, if the description of areas is 
accelerated, the directions of the forces deviate from the concourse of the 
radii towards the part to which the motion tends.” 
I have already elsewhere noticed this obscure corollary ; f 
and I only allude to it here to observe that it was scarcely to 
be expected that Newton would give much attention to the 
influence of a resisting medium as affecting his theory, (since, 
as a matter of fact, all his demonstrations are based upon the 
supposition that the heavenly bodies move in empty space,) 
and to point out the illogical character of a corollary which 
supposes the direct contrary. But Newton having drawn such 
a corollary, we need not be surprised, perhaps, that the re- 
establishment of the plenum by Encke has not disturbed the 
faith of Newton's followers in his (C demonstrations," though 
they relate only to the motions of bodies in vacuo. 
17. Before proceeding with further remarks upon current 
physical astronomy as it clashes with the teachings of the 
Principia , I would beg leave to call attention to some other 
popular astronomical dogmas. For instance, whenever we 
now look up to the heavens at night, “ to consider the moon 
* Compare Prin., b. ii., prop. 53, th. 41, Scholium. 
t Viet. Toto Codo , § 24. 
