March 19, 1908] 



NATURE 



461 



LETTERS TO THE EDITOR. 



\The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can lie itndertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for tliis or any other part of Nature. 

 No notice is taken of anonymous communications.] 



The Habitability of Mars. 

 Inasmuch as Dr. Wallace has sent me his book through 

 his publishers, as I gather from the wrapper — though it 

 is not so e.\pressed — 1 suppose it is incumbent on me to 

 acknowledge it, since he clearly e.xpects some sort of reply. 

 The effect of its perusal is to show me again how cogent 

 is the argument for the habitability of Mars, for only by 

 jnany misstatements of fact, wholly unintentional, of 

 course, can Dr. Wallace make out even a seeming case 

 upon the other side. .\ physicist will not need to have 

 these errors pointed out to him, but as most readers are 

 unable to correct them for themselves it may be wise to 

 instance a few to show how his house of cards tumbles 

 down in consequence. 



On p. 22 he quotes from Miss Gierke to prove that the 

 cap could only supply 2 inches of water over the irrigated 

 districts. Let us assume her own estimate of snow de- 

 posited, and merely correct her mathematical and topo- 

 graphic mistakes. She states the maximum area which 

 the cap covers to be 2,400,000 square miles. Now the 

 south cap comes down to 36°-5 latitude on the average, and 

 an easy calculation shows this to occupy 11,330,000 square 

 miles, or to be more than four times as great. Ne.xt, she 

 supposes the natural dark areas of the planet to be 

 irrigated, which they are not, mistaking them for the canal 

 system, which, instead of 17,000,000 square miles, covers, 

 oases and all, only about 4,750,000 according to our 

 measures, remembering that the whole of it is not watered 

 from one cap. By combining these two corrections we find, 

 not 2 inches of water for each bit of ground, but 2J feet, 

 and this according to her own estimate, which there is 

 no reason to suppose not to be two or three times too 

 small. So that it is the argument of Dr. Wallace, ar.d 

 not the cap, that fails to hold water. 



An equally fatal flaw affects Dr. Wallace's argument 

 for temperature. Here he bases his deduction on a mis- 

 statement of Prof. Poynting. Prof. Poynting states that 

 in my paper on the mean temperature of Mars I took no 

 due account of the blanketing effect of air. Not only did 

 I expressly take it into account, but I did so in the only 

 way it can correctly be taken, not by hypothesis, but by 

 direct appeal to what tal<es place on earth under a clear 

 and under a cloudy sky by night ; and I am glad to know 

 that in a paper he has sent to the Phil. .\tag. on the 

 subject Prof. Very, the bolonietric authoritv on matters of 

 temperature to-day, agrees with both my method and my 

 conclusion for Mars, and points out where Prof. Poynting's 

 calculations are fallacious. 



.Another omission is no less telling. Dr. Wallace 

 apparently is unaware that Prof. Very's bolometric deter- 

 mination of the moon's heat, which for delicacy surpasses 

 any previous ones, makes the temperature on the moon 

 during the lunar day reach 356° F. above Fahrenheit 

 zero. 



Many more such misunderstandings might be mentioned 

 occurring throughout the book, such as where, from not 

 giving its context, he makes me appear to say that water- 

 vapour is one of the heavier gases, which, of course, I 

 did not. 



Again, his theory, taken from Chamberlin, that the 

 interior of Mars can have completely lost its heat in the 

 very process of contraction, and yet later have suffered a 

 meteoric bombardment sufficient to give it a heated outer 

 layer, is mechanically whimsical, not to sav impossible. 

 For it can be shown that Mars could not have captured 

 any meteoric swarms not substantially travelling in its 

 own orbit when it coalesced into a planetary mass, and 

 any meteors subsequently encountered could onlv have 

 fallen on it as it passed through a swarm, yielding a 

 relatively insignificant amount of matter. Any such effect 

 would be even more pronounced on the earth, of the 

 occurrence of which there is no evidence. 



Misstatements cannot be too carefully avoided in science, 



NO. 2003. VOL. 77] 



especially when a man, however eminent in one branch, is 

 wandering into another not his own. Dr. Wallace, whose 

 intentions are of the highest, will appreciate this. Indeed, 

 if criticism were confined, as common-sense counsels, to 

 those versed in the phenomena, we Should hear very little 

 about the inhabitability of Mars. 



Boston, March 6. Percival Lowell. 



Dr. J. W. Evans's letter in N.ature of February 27 

 seems to invite notice from me in respect to three of the 

 subjects vifith which it deals. 



(i) As regards temperature. In most physical problems 

 temperature may be regarded as a single definite measure- 

 ment, which I understand to be Dr. Evans's point of view; 

 but this ceases to be legitimate in molecular physics when- 

 ever the behaviour of an individual molecule comes under 

 consideration. Temperature has then to be recognised as 

 not one, but many, measurements, chiefly of two groups 

 of activities, one group associated with the events that go 

 on within the molecule and are in touch witli the activities 

 of the aether, and the other group mainly concerned with 

 the journeys of the molecule through space and with one 

 section of the events that occur during each of the 

 encounters to which it may have to submit. Dr. Evans 

 will find this subject referred to, and partly dealt with, at 

 p. 76 of the Astrophysical Journal for July, 1904, or in 

 the Phil. Mag. of the preceding month. On the other 

 hand, in molar physics (as also in the kinetic theory of 

 gas as usually treated) we have no occasion to deal with 

 individuals ; we are only concerned with swarms of mole- 

 cules acting on one another and changing their behaviour 

 so frequently that the activities of or within the molecules 

 come into operation in too rapid succession to be distin- 

 guishable. All that we can then detect is that these number- 

 less activities furnish an average outcome of energy which 

 fortunately is (except in certain critical instances) suffici- 

 ently steady to admit of measurement, and is then what 

 we call the temperature. But this jumbling together of 

 unlike activities is not admissible when the question is 

 about individual molecules — as when our object is to learn 

 the conditions under which the lightest gaseous molecules 

 of an atmosphere, which are those most violently tossed 

 about, can occasionally and one by one drift away from 

 their atmosphere. 



(2) The question whether we can know that Mars is 

 unable to prevent the escape ot water is in effect almost 

 the same question as whether we may trust the evidence 

 that helium is in process of escaping from the earth, 

 inasmuch as the dynamical conditions in these two 

 problems are nearly identical. The evidence in the case 

 of helium, so far as it was known eight years ago. Dr. 

 Evans will find on pp. 369, &c., of the Astrophysical 

 Journal for June, 1900. It should be added that the dis- 

 coveries since that date about helium have materially 

 strengthened the evidence then available. 



(3) Dr. Evans bases an argument on the early state 

 of the earth, which he thinks could not have been followed 

 by the presence of water in modern times if some molecules 

 can now escape from a planet in the way I have sup- 

 posed. This, I believe, is a mistake. In the remote past 

 the potential of attraction of the dilated earth of those 

 days may have been, as supposed by Dr. Evans, so much 

 less than now that multitudes of molecvilcs now on the 

 earth were not then upon it. So much may be conceded. 

 But then, as now, these molecules were under the influence 

 of the sun's attraction, and did not range beyond a ring 

 round the sun, in which the earth also travelled — like the 

 rings of Saturn or the asteroids of the solar system. After- 

 wards, when the earth shrank and the potential of its 

 attraction rose to near its present amount, such of these 

 molecules as encountered the earth were unable to escape 

 again, and we now find them upon the earth. There is 

 therefore no such conflict as Dr. Evans supposed between 

 this possible past and the argument I have based upon 

 observed facts, viz. upon the absence of all the gases of 

 its atmosphere from the moon, and on the escape from 

 the earth of molecules of hydrogen and helium which is 

 still going on. 



The more deductive method of investigating the escape 

 of gases from atmospheres, without the premisses from 



