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 intensely heated state, and to have been parting ever since with portions of its heat, and at the same time contracting its dimensions. There are, undoubtedly, good grounds for inferring from recent observation and experiment, that the temperature of the earth increases as we descend from the surface to that slight depth to which man can penetrate: but there are no positive proofs of a secular decrease of internal heat accompanied by contraction. On the contrary, La Place has shown, by reference to astronomical observations made in the time of Hipparchus, that in the last two thousand years at least there has been no sensible contraction of the globe by cooling; for had this been the case, even to an extremely small amount, the day would have been shortened, whereas its length has certainly not diminished during that period by 1/300th of a second.