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°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°C. (=62 1/2°F.) and Mars one of −38°C. (=−36 1/2°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.'
But we know that it is far from being earth-like in the very conditions which we have found to be those which determine the extremely different tem-
