ADDRESSES 49 
stages by centrifugal action, the equatorial rotation of the 
nebula must have had successively the speeds of the separated 
parts. These speeds may indeed have been slightly altered 
subsequently by tidal and other actions, but Sir George Darwin 
has shown how trivial the tidal effects must have been at the 
most, and the others are negligible under this hypothesis. The 
velocity at the equator of the nebula must have increased from 
about 5.5 kilometers per second at the separation of Neptune 
to more than 45 kilometers per second at the separation of 
Mercury. Moreover its velocity must have continued to in- 
crease still further as the nebula shrank into the existing 
sun. As this further increase must have much exceeded that 
which had already taken place, it might well be supposed 
that material for other planets would have been separated, 
and this view was entertained in the last century and diligent 
search made for inner planets at times of solar eclipses ; indeed 
eminent astronomers even announced the discovery of such 
planets, but the observations proved illusory. If the radius 
of the orbit of such a planet had been 1,612,900 kilometers 
its orbital velocity would have been 275 kilometers per second 
and the equatorial velocity of the nebula should have been the 
same at the time of its separation. To separate matter by 
centrifugal action at the equator of the present sun, the velocity 
should be 435 kilometers per second. The actual velocity is 
about 2 kilometers per second. This is a discrepancy so en- 
ormous as to seem fatal to the centrifugal theory, 
Moreover this difficulty is supported by others not less 
formidable, though time will not permit us to consider them 
adequately. Suffice it to say that the values of the planetary 
momenta seem to be seriously at variance with the require- 
ments of the centrifugal theory. Attention was directed to 
some of these troublesome features by Babinet a half century 
ago but he does not seem to have regarded them as fatal, but 
only as limiting conditions to which the theory must conform. 
Much later and quite independently Moulton carefully in- 
quired ino the question whether the moments of momentum 
of the solar system were compatible with the centrifugal theory 
and found the discrepancies insurmountable. As his argu- 
ments rest upon the constancy of moments of momentum, a 
firmly established principle, they are scarcely less than rigorous. 
