THE UNIVERSE. 
149 
important part. In calculating the relative motions of such a 
system, the centre of gravity was taken as a point of reference, 
since this point remained fixed. It was also possible, by mathe- 
matical methods, to determine a plane such that all the forces of 
the system resolved parallel to it would give a maximum resultant. 
As this plane was invariable, it formed a convenient plane of refer- 
ence ; and just as the centre of gravity might be regarded as the 
mean point, so the invariable plane might be regarded as the mean 
plane of the system. The orbits of the bodies of the system might 
now be regarded as approximately ellipses with the centre of 
gravity as focus, which orbits at the same time oscillated about 
the mean plane. If the bodies moved in vacuo, their orbits, 
though constantly varying, would go though periodic changes, 
every phase being reproduced after certain fixed periods of time ; 
but if the bodies moved in a resisting medium, every element of 
the orbit would constantly diminish, and two marked results 
would follow. 
1. Centralization, by which all the bodies of the system would 
be constantly drawn to the centre of gravity, and so clustering 
would be developed. 
2. Applanimtion, by which the bodies would constantly tend 
towards the mean plane. 
Hence clusters, which would be primarily globular in form, 
would tend to the form of a disc. 
Observation had shown that this form held with regard to (1) 
the solar system, and (2) the stellar system, as seen in the milky 
way. The appearance of constellations in lines naturally followed 
from the constellations having assumed a disc-like arrangement, 
and man's observing these discs edgewise. 
The remainder of the lecture dealt with the probable origin of 
heavenly bodies, and their development into orders of systems. 
Arguments were adduced in support of the theory that the planets 
of the solar system had been originally united with the sun in one 
immense mass, and had been thrown off from it while cooling, and 
that by the process of centralization they would be ultimately 
drawn into it again. The theories of Wright, Kant, and Lambert, 
as to orders of suns rising in degree to one grand central sun, were 
examined, and reasons given for doubting the existence of such 
central suns, while it was admitted that the tendency of evolution 
was to develop such a system. 
