540 Constitution of Gaseous Heavenly Bodies. [September, 
from the assumption that the subsidence of the superficial 
layer during this period has always taken place with the 
speed of a freely falling body. In this manner we find, as 
an inferior limit to the time which the sun s radius wou 
have required in decreasing from an initial value equal 
about half the distance of the nearest fixed star down 
the magnitude of the radius of the orbit of Neptune, or to 
the radius of the earth’s orbit (the latter assumption giving 
a result which differs from the former merely in a vanishing 
degree), a time of about 16 million years. f 
As the inferior limit of the whole duration of existence of 
a fixed star of the mass of the sun, we should find a time ol 
about 60 million years, of which 16 million years belong 
to the nebular period, 4 millions to the transition from the 
culminating point of the heat-radiation to the culminating 
point of the temperature of eradiation, and 40 million yeais 
that part of the process of refrigeration which corre- 
sponds to the emission of light. The real duration of the 
phenomenon may be considerably greater, as in the above 
estimate the continuous increase of mass and heat detei 
mined by the fall of meteorites has been ignored. 
[It will at once strike the reader that this estimate does 
not agree with an earlier passage in this memoir, where the 
life of our sun to the end of its luminosity, and appaient y 
without including the nebular portion of its career, is taken 
at 76 million years. Geologists and biologists will require 
much more positive evidence than astronomers and phy- 
sicists have yet furnished before accepting such low esti- 
mates for theage of the sun, and consequently of the earth.j 
As a confirmation of the theory here put forward may 
nerhaps be noticed the change in the colour of Sinus which 
has occurred within historical times. That this star was 
decidedly red 2000 years ago can scarcely be doubted aftei 
the accordant evidence of Ptolemy, Cicero Horace and 
Seneca. Seneca remarks expressly that the light of Sinus 
was more decidedly red than that of Mars As Sinus now 
pnnears of a bluish white it would have to be assumed, ac- 
cording to the author’s theory, that about 2000 years ago 
Sirius was still in the first stage of development of red 
light, and that its temperature of eradiation has considei- 
ably increased in the meantime. . ( .l 
On the assumption of a certain mean consistence of the 
«as of which our Sun consists, by applying the above method 
of investigation we should arrive at the result that its ladius 
has required 28,150 years in decreasing from 100 times to 
20 times its present magnitude, and that duiing this time 
