evolutions, in terrestrial
climates, he says, that "it is a matter of observed fact, that many
stars _have_ undergone, in past ages, within the records of astronomical
history, very extensive changes in apparent lustre, without a change of
distance adequate to producing such an effect. If our sun were even
_intrinsically_ much brighter than at present, the mean temperature of
the surface of our globe would, of course, be proportionally greater. I
speak now not of periodical, but of secular changes. But the argument
is complicated with the consideration of the possibly imperfect
transparency of the celestial spaces, and with the cause of that
imperfect transparency, which may be due to material non-luminous
particles diffused irregularly in patches analogous to nebulae, but of
greater extent--to _cosmical clouds_, in short--of whose existence we
have, I think, some indication in the singular and apparently capricious
phenomena of temporary stars, and perhaps in the recent extraordinary
sudden increase and hardly less sudden diminution of [Greek: e]
_Argus_."[209]

More recently (1852) Schwabe has observed that the spots on the sun
alternately increase and decrease in the course of every ten years, and
Captain Sabine has pointed out that this variable obscuration coincides
in time both as to its maximum and minimum with changes in all those
terrestrial magnetic variations which are caused by the sun. Hence he
infers that the period of alteration in the spots is a _solar magnetic
period_. Assuming such to be the case, the variable light of some stars
may indicate a similar phenomenon, or they may be stellar magnetic
periods, differing only in the degree of obscuration and its duration.
And as hitherto we have perceived no fluctuation in the heat received by
the earth from the sun coincident with the _solar magnetic period_, so
the fluctuations in the brilliancy of the stars may not perhaps be
attended with any perceptible alteration in their power of radiating
heat. But before we can speculate with advantage in this new and
interesting field of inquiry, we require more facts and observations.

_Supposed gradual diminution of the earth's primitive heat._--The
gradual diminution of the supposed primitive heat of the globe has been
resorted to by many geologists as the principal cause of alterations of
climate. The matter of our planet is imagined, in accordance with the
conjectures of Leibnitz, to have been originally in an