that the conditions under which life is now maintained on the earth are derived from the additions which the solar radiation constantly makes to the intrinsic heat of our planet. It becomes then a point of the highest interest to ascertain the actual temperature of this great ruler of our system, what its present rate of cooling would be if that temperature were not continually restored, and what means of maintaining that temperature appear to exist. It is with the first of these that we have to do, but it may not be without interest to state here one of the most probable estimates that has been made as to the other two. Ericsson has calculated that, if the contraction which constant radiation of heat must necessarily effect in the sun, should proceed at the rate of one foot of his radius in 3 days and about 34 ½ minutes (3.024 days), the development of heat would equal the radiation from the solar surface, as he determines it. Contraction at this rate would in about 2,000,000 years reduce the sun's diameter by one-tenth. The thermal energy at the sun's surface would, during the whole period, be maintained constant at its present figure, but the diminished size of the solar disc would result in a diminution of the temperature communicated to the earth, at its present distance, by an average of nearly 13° Fah. Although this would involve a notable change in the conditions of life on the earth, and would probably be sufficient to depopulate its arctic regions, the change would not reduce the average temperature of tropical regions to that which at present prevails in Dunedin. This is, of course, all pure speculation, because the data assumed cannot be verified. My object to-night is indeed to show that they are very unreliable. We may, nevertheless, rest content in the conviction that the secular cooling and contraction of the sun, if it should actually be proceeding in the manner which Ericsson assumes, will have no effect on the conditions of life upon the earth within a period utterly beyond our powers of conception; for, though we can express millions of years in numbers, the idea which we form in our minds of such a lapse of time is really of the vaguest description. At any rate, there appears to be every reason to believe that before the earth becomes too cold to be inhabited by beings like ourselves, there will be ample time for “social evolution” to carry our race to that future of perfect development towards which we are taught to believe that it is inevitably tending. The subject we have in hand excited, not long ago, considerable attention amongst scientific men, on account of the publication by Father Secchi, in his work, “Le Soleil” (Paris, 1870), of his estimate of the solar temperature as not less than 10,000,000° Cent. (say 18,000,000° Fah.) The learned Director of the Roman Observatory, in the calculations by which he arrived at this enormous temperature, rejected the received law of radiation estab