the production of these changes, but from among these causes we may exclude the variations in the sun's heat. There does not seem to be the least reason to suppose that any alteration in the rate at which the sun diffuses heat has been a cause of the vicissitudes of climates which the earth has certainly undergone within geological times.
And yet we feel certain that the incessant radiation from the sun must be producing a profound effect on its stores of energy. The only way of reconciling this with the total absence of evidence of the expected changes is to be found in the supposition that such is the mighty mass of the sun, such the prodigious supply of heat, or what is equivalent to heat, which it contains, that the grand transformation through which it is passing proceeds at a rate so slow that, during the ages accessible to our observations, the results achieved have been imperceptible. Think of a sphere the size of the earth. Would it be possible to detect the curvature of a portion of its equator a yard in length? To our senses, nay, even to our most refined measurements, such a line, though indeed a portion of a circular arc, would be indistinguishable from a straight line. So is it with the solar radiation. To our ephemeral glance it appears to be quite uniform; we can only study a very minute part of the whole series of changes, so that we are as little able to detect the want of uniformity as we should be to detect the departure from a straight line of the arc of a circle which we have given as an illustration.
We cannot, however, attribute to the sun any miraculous power of generating heat. That great body cannot disobey those laws which we have learned from experi-
