cavernous rock-texture, leading to extremely rapid radiation of heat in the one; as compared with a comparatively even and well-compacted surface largely clad with vegetation, leading to comparatively slow and gradual loss by radiation in the other:  and (2), these results being greatly intensified by the total absence of a protecting atmosphere in the former, while a dense and cloudy atmosphere with an ever-present supply of water-vapour, accumulates and equalises the heat received by the latter.

The only other essential difference in the two bodies which may possibly aid in the production of this marvellous result, is the fact of our day and night having a mean length of 12 hours, while those of the moon are about 14-1/2 of our days.  But the altogether unexpected fact, in which two independent enquirers agree, that during the few hours’ duration of a total eclipse of the moon so large a proportion of the heat is lost by radiation renders it almost certain that the resulting low temperature would be not very much less if the moon had a day and night the same length as our own.

The great lesson we learn by this extreme contrast of conditions supplied to us by nature, as if to enable us to solve some of her problems, is, the overwhelming importance, first, of a dense and well-compacted surface, due to water-action and strong gravitative force; secondly, of a more or less general coat of vegetation; and, thirdly, of a dense vapour-laden atmosphere.  These three favourable conditions result in a mean temperature of about +60 deg.  F. with a range seldom exceeding 40 deg. above or below it, while over more than half the land-surface of the earth the temperature rarely falls below the freezing point.  On the other hand, we have a globe of the same materials and at the same distance from the sun, with a maximum temperature of freezing water, and a minimum not very far from the absolute zero, the monthly mean being probably much below the freezing point of carbonic-acid gas—­a difference entirely due to the absence of these three favourable conditions.

The Special Features of Mars as influencing Temperature.

Coming now to the special feature of Mars and its probable temperature, we find that most writers have arrived at a very different conclusion from that of Mr. Lowell, who himself quotes Mr. Moulton as an authority who ‘recently, by the application of Stefan’s law,’ has found the mean temperature of this planet to be-35 deg.  F. Again, Professor J.H.  Poynting, in his lecture on ‘Radiation in the Solar System,’ delivered before the British Association at Cambridge in 1904, gave an estimate of the mean temperature of the planets, arrived at from measurements of the sun’s emissive power and the application of Stefan’s law to the distances of the several planets, and he thus finds the earth to have a mean temperature of 17 deg.  C. (=62-1/2 deg.  F.) and Mars one of-38 deg.  C. (=-36-1/2 deg.  F.), a wonderfully close approximation to the mean temperature of the earth as determined by direct measurement, and therefore, presumably, an equally near approximation to that of Mars as dependent on distance from the sun, and ’on the supposition that it is earth-like in all its conditions.’