350 D. Trowbridge on the Nebular Hypothesis. 
tion of equilibrium. The conditions of equilibrium are these: 
the fluid matter must be arranged in strata the density of which 
throughout must be the same. These strata may differ from 
each other in density, or the density of each one may be the 
same—that is, the body may be homogeneous, Also the result- 
ant of all the forces acting on any one of these strata, must be 
directed towards the interior of the mass, and be normal to all 
the strata.” 
the time of rotation will be reduced to 2h 25m 26s.* If the pe- 
riod of rotation becomes still less, the fluid mass can no longer 
hold together; in other words, the equilibrium will no longer 
exist. 
16. It has never been proved, so far as I am aware, that when 
the body is heterogeneous, there are more than one form of 
equilibrium, If the rotatory motion be slow, the demonstrated 
form is one of small ellipticity.* But if it never has been 
roved that there are more forms of equilibrium of a revolving 
eterogeneous mass, than one, it is, perhaps, nearly certain that, 
at least, two forms exist for the same period of rotation. Isit 
more reasonable to assume, as has been done, that the very ob- 
late form is a little less oblate than the corresponding form of & 
homogeneous spheroid? Since the form of small ellipticity 18 
fess oblate in a heterogeneous spheroid than the correspondin 
form of a homogeneous one, does it not seem probable that the 
very oblate form of the former, is still more so than the similar 
one of the latter: 
17. We shall now assume that the primitive rotating solar 
mass was, at least, mostly an aériform fluid body, and that i 
a 
a ; 
approximated to a very oblate spheroid. Before proceeding 
of a stratum, t a particle on this surface has no tendency to move in 
irection, pote Pareon have DsV =0= the force acting on the particle. Inte 
Grating aod V = B= 
gs Airy’s Tracts, 4th ed., p. 148. * Tb. p. 149. 
_ _* Ib., pp. 150-175; Pratt’s Fig. of the Earth, pp. 63-79; Math. Monthl 
fii, pp. 166-182. The problem of 
of small ellipticity. 
7 
the figure of a heterogeneous earth is one © 
astronomy, and it has been melred onl be OS 
