&8  Correspondence—Mr. S. T. Preston. 
If the earth (completely crusted over) have existed “far more than 
twice 20 million years,” a fortiori then must the sun have been 
present longer than this period; for it is obvious that a habitable _ 
globe could not survive without the sun. But it is certain that if 
the sun have endured anything like this time, the contained store of. 
heat must have been such—as Dr. Croll points out—that the solar 
nebula at that primitive age ‘would have extended beyond our 
Harth’s present orbit, and of course our Harth could not at that time 
have existed as a separate planet” (Dr. Croll’s paper in the Philo- 
sophical Magazine, May, 1868, p. 872). To infer the degree of 
expansion of the sun at that remote epoch; we have merely to deduce 
(an easy process) what the sun’s temperature then was at his present 
rate of cooling. 
Dr. Croll does not attempt to solve the difficulty he lays stress on; 
but is it not obvious, in view especially of the recent ideas as to 
space being’ comparable somewhat to a “‘ meteoric plenum,” not to 
mention the continued other friction opposed to the flying globe— 
that the Harth’s present orbit could not have been its original orbit ? 
But the Harth must have come in an enormous distance towards the 
sun in over twice 20 million years, which is the lowest limit for 
the period which geology demands for the Harth’s past existence, 
according to the evidence afforded by the known rate of deposition 
of sedimentary strata, ete. This kind of evidence is not vague, but 
mathematically convincing. Dr. Croll remarks, viz. “ We have not 
sufficient data to determine how many years have elapsed since life 
began on the globe, for we do not know the total amount of rock 
removed by denudation; but we have data perfectly sufficient to 
show that it began far more than twice 20 million years ago” 
(Quarterly Journal of Science, July, 1877, p. 317). 
So then we have the apparent fitness that then the sun was hotter 
than at present, the planets were further off. As the sun cools 
down, the planets approach him, this fact equalizing the conditions 
for life on the Earth over a far longer time-epoch than would other- 
wise be possible. This consideration affords apparently plenty of 
margin for past geological time, without coming into any conflict 
with physics. 
The idea of a 20 million years’ margin for the age of the sun’s 
heat, enunciated by Lord Kelvin, depends on the assumption (in 
aspect arbitrary?) that the sun was formed by the gravitational 
approach of widely diffused matter in a primitive state of rest, for 
which ‘state of rest” we have surely no evidence. On the con- 
trary, all analogy (all we observe in the skies now) goes to indicate 
that the matter whose collision formed the sun, was originally in 
motion. If this primordial motion naturally contributed to the heat 
developed at the concussion of such moving matter, whose encounter 
generated the solar heat: then any store of heat that geological time 
may require, could have been irrefragibly produced at the sun’s 
formation. 
' In a letter in ‘‘ Nature,’’ March 28, 1878, p. 248, on “The Age of the Sun’s 
Heat in relation to Geological Evidence,’’? an analogous solution to the above 
difficulty was offered by the present writer. 
