382 
Royal's Lectures, and a3 accepted in tlie Royal Astronomical 
Society, is not actually in accordance with anything that pur- 
ports to be demonstrated in Newton's Principia ! Let me 
not be misunderstood. I am not saying that our modern 
astronomers do not profess to believe in Kepler's laws and 
in Newton's theory and demonstrations. But I do say, that 
whatever they may vaguely and inconsistently profess, they 
do not hold Newton's conclusions, and that the conclusions 
he has professed to establish are not in accordance with what 
is now believed. And yet I am bound to add further, how'- 
ever paradoxical it may sound, that Newton is in a certain 
sense responsible for even what the moderns believe, though 
discordant with his professed demonstrations, and not in 
accordance with what either he himself or Copernicus or 
Kepler believed. 
7. Let me now endeavour to reconcile and explain these 
apparently conflicting assertions. In the first place, we are 
all accustomed vaguely to speak of our believing in the truth 
of the Copernican system of astronomy as opposed to the 
Ptolemaic ; but we do not literally believe what Copernicus 
taught, namely, that the sun is at rest in space, and that the 
orbits of the earth and planets round the sun are circular. 
Then, again, we still talk of believing in the truth of Kepler's 
laws of the elliptical orbits of the earth and planets round the 
sun ; but Kepler, too, believed the sun to be at rest, though 
not in the precise centre of the planetary orbits. And yet we 
ought to remember that an ellipse as well as a circle is a curve 
that returns into itself, and that no ellipsis can possibly be 
described round a moving centre or focus that is travelling 
rapidly onwards in space ; but this, according to Professor Airy, 
is now believed, as regards our sun, “ by every astronomer 
who has examined the question carefully." * 
8. Once more. Sir Isaac Newton, in the Principia, professes 
to establish upon a mathematical basis what Kepler taught 
were the motions of the heavenly bodies, superadding a phy- 
sical cause or Jaw to account for those motions after they have 
once been set agoing. That law, as is well known, was gravi- 
tation. The theory of universal gravitation (as I have already 
stated in this Institute) was previously propounded by Halley 
and Hook to the Royal Society of London, ten and twelve 
years before the Principia was published.! How the original 
conception of the theory came to be assigned to Newton, and 
the mythical story of his apple to be invented, I do not know ; 
* Airy’s Lects. on Astron. (4tii edit.), p. 173. 
t Vide Journ. of Trans, of Viet. Institute , vol. i. pp. 413, 414 ; and Phil. 
Trans, there cited, vol. ii. pp. 126, 127, and 326. (Lond. 1809.; 
