1904-5.] On Prof. Seeligers Theory of Temporary Stars. 549 
ably and suspiciously irregular ; and that there is an unexplainable 
anomaly in the motion of the inner ring of Saturn. 
‘‘Under the methods of the second category, it is shown that 
the development of a system of planets and satellites from an 
extended nebula is by no means a simple matter, and that in the 
system under consideration the conclusions which it was possible 
to make were invariably adverse to the theory. In subjects where 
perfectly rigorous mathematical processes cannot be employed, 
such a uniform agreement of conclusions, when so various methods 
of attack are employed, is sufficient to establish a proposition. 
The objections are, that the lighter elements would have escaped ; 
that matter would have been left off continually, instead of in rings 
at rare intervals ; that if a ring were all contracted into a planet 
except an infinitesimal remainder distributed in its path, the 
process of aggregation could not complete itself ; that the gravita- 
tion between the masses occurring in the rare media would be so 
feeble that they would seldom come in contact, and that Roche’s 
limit and a similar new criterion show that fluid masses of the 
density which must have existed would be disintegrated by the 
disturbing action of the sun. 
“ The one objection wffiich is advanced in the methods of the 
third category * is of great simplicity, and leads to certain conclu- 
sions. It is of such a character, and the numerical discrepancies 
are so great, that it seems to render the nebular hypothesis, in the 
simple form in which it has usually been accepted, absolutely 
untenable, unless some fundamental postulates, now generally 
* (P. 126.) “ It is known from the elementary principles of dynamics that 
the moment of momentum of a system which is subject to no external forces 
is constant.” Mr Moulton demonstrates, however, that when the solar 
nebula extended to Neptune’s orbit, the moment of momentum was 32 ’176, 
while in the system at present it is only 0 *1 51. Hence, “instead of being a 
constant, the moment of momentum is found to vary in a remarkable 
manner. ... It follows from these figures that if the mass of the solar 
system filled a spheroid extending to Neptune’s orbit, and rotated with a 
velocity sufficient to make its moment of momentum equal to that of the 
present system, and if it then contracted .... the centrifugal force would 
not equal the centripetal until it had shrunk far within Mercury’s orbit. 
Such an enormous difference cannot be ascribed to uncertainties in the law 
of density, or to the approximations in the mechanical quadratures ; but it 
p nnts to a mode of development quite different from, and much more com- 
plicated than, that postulated in the nebular theory under discussion.” 
