D. Kirkwood—Solar and Sidereal Heat, 298 
against the other. Comets pass rownd the sun without collision 
containing one-half the matter of the solar system, were ap- 
proaching one another in the same straight line, each at the 
rate of 1,70 miles per second ;* that on meeting, their motion 
was transformed into heat; and that their united mass was at 
once reduced to vapor: the great question yet remains—How 
much of the period represented by these 800,000,000 years’ heat 
can be claimed as geological time? The nebula formed by the 
collision would extend far beyond the present orbit of Nep- 
tune. The amount of heat radiated in a given time from so 
vast a surface would doubtless be much greater than that now 
emitted in an equal period. No considerable contraction could 
occur until a large proportion of the heat produced by the 
’ 
inevitable that much the greater part of the 800,000,000 years 
pon the whole, it seems more difficult to grant the demands 
of Dr. Croll’s hypothesis than to believe that in former ages the 
stratification of the earth’s crust proceeded more rapidly than 
at presen ormer, as we have seen, has no sufficient 
basis in the facts of observation. On the other hand, if our 
planet has cooled down from a state of igneous fluidity, the 
great heat of former times must doubtless have intensified b 
aqueous and atmospheric agencies in producing modifications 
of the earth’s exterior. 
Bloomington, Indiana, March, 1878. 
* This i ity mentioned by Dr. Croll. An 
mation would of pase Ras lB more heat, but the hypothesis 
+ Proc. Kan Phil 860, vol. xvi, pp. 329-333 and National Quarterly Review, 
March, 1877, p. 292. 
Am. Jour. wijees dace ya: Vou. XV, No. 88.—APRIL, 1878. 
increased rate of 
would be open to 
