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mathematical mind would have enabled him to devise processes by means of 
which that mathematical system of reasoning could have been improved, 
and made to bring in facts which are now supposed to be brought into 
physical astronomy 
Mr. Reddie. — Do you mean theoretical or practical astronomy ? 
The Chairman. — What I mean is this. There is a difference between 
what is technically called plain astronomy, which deals with the actual, 
visible motions of the planets, and that which accounts for motions that are 
matter of theory. This is called physical astronomy, and it consists in the 
main of two parts — namely, what is called the lunar theory, and which accounts 
for the exceedingly complicated apparent motions of the moon with respect 
to the fixed stars ; and the planetary theory, which accounts for the equally 
complicated motions of the planets. Physical astronomy does not go much 
beyond these two theorems : the apparent motions of the moon amongst the 
stars, and the apparent motions of the planets amongst the stars ; and ex- 
tremely complicated motions they are. If you traced them on the celestial 
globe, you would find that they described curves of the most complicated 
character. It is the business of physical astronomy, on the hypothesis of 
gravitation, not only to account for these extremely complicated motions 
but to do more than this : to predict the position of these bodies, and tell 
where they will be at any future time. The mathematical astronomers were 
for a long time bigoted to the processes of Newton, and while they were physical 
astronomy made no progress in this country. (Hear, hear.) Astronomers who 
were not prejudiced, however, took up the methods of Leibnitz, and the conse- 
quence was that they were able to predict the motions of the moon amongst 
the stars. And therefore, supposing that the whole Principia was abolished 
at once — if it were given up, you have not attacked current physical 
astronomy, because it does not the slightest degree depend upon Newton’s 
Principia, or any proposition in that Principia, except the assumption of 
the three laws of motion. Current physical astronomy is based upon the 
assumption of the law that Newton determined — namely, that of gravitating 
bodies attracting one another directly as their masses and inversely as the 
square of their distances. Take that hypothesis for granted, and combine it 
with the three laws of motion — not one of which can be obtained from 
direct experiment, but which are derived incidentally from thousands of 
experiments, and deduced rather than proved, — and the physical astronomer 
maintains that he can predict these exceedingly complicated motions, so as 
to account for the positions of the heavenly bodies with an extreme degree 
of accuracy. The question is, can the physical astronomer do so ? You can 
attack him in two ways. You can show that his mathematical analysis is 
unsafe, and not fit to be trusted ; or you can show, which is still more 
important, that he cannot calculate these things beforehand — that observation 
does not agree with his theory. I think that is the way in which current 
physical astronomy is to be opposed, and not in the manner Mr. Reddie has 
done. For if he has done anything at all, all that he has done is to oppose 
Newton’s mode of demonstration and mode of reasoning in the Principia. 
