HYPOTHESES BEARING ON CLIMATIC CHANGES 669 
the process would be slow; the individual conjunctions more or 
less distant in time, and the heat generated by one impact so far 
forth lost before another took place; second, the conjunctions 
would not be opposing collisions but overtakes in which both 
bodies were moving at nearly the same speed, and the heat of 
conjunction hence relatively small. It would appear, therefore, 
that the aggregation might take place without the development 
at any one time of a general high temperature. The present 
accretions of the earth show us that growth is possible without 
notable increase of temperature. Following the general line of 
the nebular hypothesis, therefore, it 1s possible to suppose the 
earth to have been affected by relatively low surface temperatures 
at all stages of its growth. By changing our assumptions as to 
the rate and vigor of accretion we can correspondingly change 
our conclusions respecting the earth’s temperature. The range 
geologist, because it involves a supposed objection to meteoroidal aggregation. Ina 
solid rotating ring the outer part moves faster than the inner and if broken and con- 
densed to the globular form the rotation must be direct. But in a ring of planetoids 
the inner members move faster than the outer and if the several concentric orbits be 
symmetrically drawn together so that the inner planetoids uniformly or usually collide 
with the inner sides of the outer planetoids retrograde rotation follows. But this is 
inconsistent with the facts of the solar system except in the case of Uranus and 
Neptune (Cf. Faye, sur !’Origine du Monde, 1896, pp. 165, 270-281). But it seems 
improbable that this would be the mode of union except in the case of the outer planets, 
for the mutual gravitation of minute planetoidsis very slight and expresses itself chiefly 
in perturbations under such conditions (see On the Stability of Motion of Saturn’s 
Rings, Scientific Papers of James Clerk Maxwell, Vol. I, pp. 288-376), while the disturb- 
ing influence of the great planets is appreciable, as the ellipticity of the orbits of the 
planets testify. If the orbits of the particles or planetoids of the supposed earth- 
moon ring were at first nearly circular and concentric the conjoined attractions of 
the outer planets would render them elliptical. But the line of their apsides would 
not be concordant and would be subject to subsequent shifting in a more or less 
non-concordant fashion. It is therefore conceived that they would be brought to 
cross each other and that this would lead to collisions. Now an outer orbit could 
only cross an inner one by a more or less perihelion portion of it coming into coinci- 
dence with a more or less aphelion portion of the inner one. But the perihelion 
movement of a body in an outer orbit is greater than the coincident aphelion move- 
ment of a body in an inner orbit. Hence on the average the outer body in collision 
will have the greater speed and the consequent rotation wiil be direct. As this 
reasoning applies to the inner planets and not to the outer, and as the inner planets 
have direct rotations while the outer probably have retrograde rotations, it has at 
least the merit of coincidence with the facts. 
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