8 SCIENCE PROGRESS 



artificial means from the polar regions. The theory is there- 

 fore quite untenable, unless the maximum temperature on Mars 

 is well above the melting-point of ice. There are two out- 

 standing determinations of the mean temperature, and the 

 great difficulty is that they appear to be absolutely contradictory. 

 The first is due to the late Prof. Poynting, 1 who, from a dis- 

 cussion of the general properties of solar radiation, finds the 

 mean temperature of Mars to be — 38 C. ; the second is that 

 of Prof. Lowell 2 himself, and leads to the value + 8° C. The 

 discrepancy, though less than 50° C, affects the whole question, 

 for if Prof. Poynting's value is correct, it is quite certain that 

 on Mars ice will never melt. 



It is easy enough, under certain assumptions, to calculate 

 the mean temperature of Mars from that of the earth. Assuming 

 the two planets to be similar in their behaviour towards solar 

 radiation, and ignoring the central heat, we have that the 

 energy received from the sun is inversely proportional to 

 the square of the distance, while the energy given out is, by 

 Stefan's Law, directly proportional to the fourth power of 

 the absolute temperature. But since the temperature does not 

 vary much, the energy received must balance that radiated out, 

 so that the fourth power of the absolute temperature is inversely 

 proportional to the square of the distance from the sun ; or, 

 in other words, the absolute temperature is inversely pro- 

 portional to the square root of the distance. Taking the ratio 

 of the distances as 1*5237, and the mean temperature of the 

 earth as i5°C, we get that of Mars as — 39 C, practically 

 that obtained by Prof. Poynting. 



Now we have here made the assumption that Mars and 

 the earth are similar in their behaviour towards solar radiation. 

 This is what Prof. Lowell denies. 



The solar radiation on a planet may be either reflected or 

 absorbed. The reflected radiation plays no part in raising 

 the temperature. " Strange to say," remarks Prof. Lowell, 3 

 this important fact had never been taken into account till the 

 present investigation of the subject, which led to a completely 

 different outcome from what had previously been supposed." 



1 " Radiation in the Solar System : its Effect on Temperature and its Pressure 

 on Small Bodies," Phil. Trans. A, 202 (1903), p. 525. 



3 Mars as the Abode of Life (New York, 1909), pp. 240 et seq. 

 3 /bid. p. 83. 



