April 28, 1S93.J 



SCIENCE. 



233 



probably strengthen rather than obliterate the weather curve, not assume to offer a solution for this complex problem, but hope 



especially when we consider the effect of increasing vegetation rather to encourage discussion which will call out all the princi- 



which would follow increased rainfall. L. E. Hicks. pies of physical science applicable to it. W. H. Howard. 



Lincoln, Neb., Nov. 4, 1893. Adrian College, Adrian, Mich., April 15. 



The Moon's Atmosphere. 



In Science of Feb. 24, Sir Robert Ball makes application of the 

 kinetic theory of gases to explain the absence of air from the 

 moon. He observes that, although the mean molecular velocity 

 of translation is less than that required by a body projected verti- 

 cally from the moon to overcome the moon's attraction, " in the 

 course of their movements, individual molecules frequently 

 attain velocities very much in excess of the average pace, " and 

 would therefore be able to escape from the moon into space, and 

 thus, in time, the whole atmosphere would be lost. I think a 

 full consideration of the subject will not justify that conclusion, 

 but that we shall be obliged to resort to some other physical laws 

 to solve this old problem of speculation. 



The kinetic theory requires all the molecules of a gas to have 

 equal ma-^ses, equal energies, and hence equal mean velocities. 

 This mean velocity for the hydrogen molecules at 0" C. is about 

 1,800 metres per second, while that of oxygen and nitrogen is 

 abaut 450 metres per second, since the velocity is inversely pro- 

 portional to the square root of the mass of the molecule. To 

 overcome the moon's attraction a body must have a vertical 

 velocity of about 2,300 metres per second. But it must be re- 

 marked that the escaping molecules, if ther'e are such, are only 

 those of the outer confines of the atmospheric envelope, where 

 the mean free path of the molecules is relatively very great, as 

 suggested with respect to the earth's atmosphere by H. Daniells 

 ("Principles of Physics"), and the temperature of those regions 

 is very low. If the temperature is about 68° absolute scale 

 (—204° C), as assumed by some authorities, the mean molecular 

 velocity falls to about 325 metres a second, since the velocity 

 varies as the square root of the absolute temperature. The verti- 

 cal velocity, then, or the vertical component of the velocity must 

 be about ten times the mean velocity to balance the force of gravi- 

 tation, which is not probable. 



Again, if the temperature is much lower than 68' absolute, 

 approximating the absolute zero, and the molecular velocity always 

 obeys the law before mentioned, the velocity also would aiiproxi- 

 mate zero, and of course the molecules could not escape the at- 

 traction. It appears, then, to be largely a question of the tem- 

 perature of the outer limits of an atmosphere. "With this in view, 

 let us compare results on planetary bodies of different size and 

 stage of world life. As already suggested, with respect to the 

 earth and moon, the earth's attraction at the surface is about five 

 times that of the moon at its surface. This, eceteris paribus, 

 would require about five times greater molecular velocity of its 

 atmosphere to escape than for that of the moon. But, if we take 

 into account the previous history of the two bodies, it is observed 

 that the earth was highly heated for ages after the moon had be- 

 come comparatively cool, and this must have rarefied and expelled 

 its atmosphere to great heights, and maintained a tempei'ature in 

 those regions which, according to the proposition under dis- 

 cussion, would have caused the earth to lose its atmosphere. In 

 general, it would follow that the major planets and larger satellites 

 would lose their atmospheres more completely while cooling than 

 the smaller ones, unless they have correspondingly greater quan- 

 tities of volatile matter in their composition than the smaller 

 ones. And sucli, seems to be the result. Even Jupitei', whose 

 attraction at the surface is 2.6 times that of the earth, is believed 

 to have an atmosphere much less extensive proportionately than 

 the earth. Mars offers a good example of a small planet with a 

 copious atmosphere. Its attraction is only about twice that of 

 the moon. Why has he not lest his atmosphere? If the applica- 

 tion of the kinetic theory alone explains the loss of the moon's 

 atmosphere, it would require Mars to have suffered the sam^ate 

 before now. Possibly we are committing the error of the (jBbek 

 philosophers in treating melecules as independent masses instead 

 of regarding them as inter-dependent centres of activity whose 

 pheni) nena, as a system, constitute the qualities of matter. I do 



Note on the Crystalline Lens of the Eye. 



Mr. McLouth's observation upon "A Peculiar Eye," as ob- 

 served by him in "a domestic animal," given in Science, No. 531, 

 would have been considerably enhanced in value had he recorded 

 at the same time what that " domestic animal" was; whether it 

 was an anserine fowl, as a duck or goose; or a gallinaceous one, 

 as a hen, turkey, peacock, or guinea-fowl; or whether a car- 

 nivorous mammal, as a dog, or a cat ; or an Equus, or a Bos, or a 

 Sus, or an Ovis, or what not. 



To the minds of some, the so-called "domestic animals " form 

 a natural group, and even such an authority as Girard was so 

 blind as once to propose a special classification for the domesti- 

 cated mammals! It is not uninteresting to trace the origin of 

 this idea, associated as it is in a way with the kindred one of man 

 holding a place apart from the rest of organized beings. 



It is only necessary to invite Mr. McLouth's attention here to 

 the fact that the crystalline lens in the eye of man consists of 

 three triangular segments, and their existence is easily demon- 

 strated by immersion of the lens in strong alcohol, or by boiling 

 it. The apices of these three segments are at the centre of the 

 lens, in front; their bases in the circumference. Another 

 structural feature of the lens is seen in the laminae of which 

 it is composed. The treatment just proposed demonstrates 

 these also, consisting, as they do, of concentric layers, 

 which are firm at the centre, but become softer as we ap- 

 proach the peripheral ones. Likewise, by thus treating the 

 crystalline lens from the eye of a horse, we prove that it also 

 divides into its concentric laminse, and its three triangular seg- 

 ments. But whether this holds true in the case of all vertebrates 

 has not, I think, been demonstrated. Very likely the crystalline 

 lens of the "domestic animal" examined by Mr. McLouth had 

 been submitted to a process which had a similar effect upon it as 

 boiling or immersion in alcohol would have had, and simply ex- 

 hibited its normal structure. From what I can gather from the 

 communication of your correspondent in Science there was noth. 

 ing abnormal about the lens of the eye he examined. 



R. W. Shufeldt. 



Takoma, D.C., April 14. 



The Aurora. 



In Science for April 7, at page 186, certain statements of mine 

 in regard to auroral effects proceeding from the sun's eastern 

 limb are called in question. It would have been much more sat- 

 isfactory if these criticisms had given evidence of such familiarity 

 with the subject as would be shown by the mention of even a 

 single date on which it might be claimed that an aurora appeared 

 in the absence of well-defined solar conditions of the character 

 indicated. Except where specific mention is made of such indi- 

 vidual instances, the writer proposes to refrain from discussion, 

 which would readily become interminable as well as utterly in- 

 conclusive. Such results as those of Professor Ricco, recently 

 announced in Astronomy and Astro-Physics and elsewhere, it is a 

 pleasure to meet with and comment upon. He simply takes the 

 case of the great magnetic storms of 1893, which were eleven in 

 number, and studies the coincident solar conditions, especially 

 with reference to the location of spot groups at the meridian. In 

 seven out of the eleven instances he finds that there were such 

 groups on the meridian, but that the magnetic effect, if it pro- 

 ceeded from them at all, was not felt for a varying period of from 

 twenty-one to fifty-one hours subsequently. If, however, he had 

 gone further and inquired what there was at the eastern limb on 

 these dates, he would have found that there was a spot group in 

 that location in every one of these instances without any excep- 

 tion whatever, and that these groups were located upon areas 

 which were much disturbed at successive returns by rotation. 

 Moreover, there was in these instances no appreciable retardation 

 or variability of retardation, the magnetic storm being in progress 



