Nov. 2, 1 871] 



NATURE 



this we find some pertinent and useful hints on the best 

 mode of grouping hardy perennials, and the art of 

 managing the rock-garden, the wild-garden, water, and 

 Ijoggy gro'ind ; on tlie culture and propagation of early 

 tlotvers, and other subjects de.ir to the dweller in the 

 country. Compared wilh the art of gardening as practised 

 twenty vears ago, we arc certainly now in an altogether 

 new and improved epoch, and Mr. liobinson is one of the 

 pioneers to whom we are mainly indebted for the intro- 

 duction of a belter and more rational style. A. W. B. 

 Hints on Shore-Shooting: t^'ilh a cliaptcr on skinning 

 and preserving Biids. By James Edmund Harting, 

 F.L.S., S:c. (Xordcn : Van Voorst, 1871.) 

 A GOOD sportsman, whether he knows it or not, must be 

 more or less of a good naturalist, and this Mr. Harting is. 

 His unpretcnding'little book, therefore, certainly deserves 

 mention here, and the mo-e so since he has worthily won 

 his spurs by making the group of birds most sought by 

 the " shore-shooter" ?.n especial subject of study. What 

 he tells us is the result of his own observation, and is 

 pleasantly told. What he does not tell us is whether 

 " shore-shooting " has, with most people,— for we except 

 him— any other raison dctre than the " fine-day-let's-go- 

 andkill-something" impulse. If not, we really donotsee 

 that there is much difference in principle between Pagham 

 and Hurlingham. 



LETTERS TO THE EDITOR 



[ Th€ Editor does not hold himself responsible for opinions expressed 

 by his correspondents. No notice is taken of anonymous 

 commnnications. ] 



An Universal Atmosphere 

 I HAVE much pleasure in replying to Mr. Browning's question 

 in Naturk, vol. iv. p. 4S7, as it is one that legitimately 

 strikes at the root of all my .speculations, and which, if un- 

 answerable, conveys an objection th.at must demolish the whole 

 structure I have endeavoured to erect in my essay on the " Fuel 

 cf the Sun." 



If I am right, the atmospheres of the sun, the moon, the 

 planets, or ot any other cosmical body of known mass and 

 dimensions, may be calculated in units of the earth's total atmo- 

 sphere by the simple formula reasoned out in Chap. iii. of the 

 above-named work, i.e.. by multiplying the mass of the body 

 (expressed in units of the earth's mass) by its own square root, 

 thus 



X = m \l m ; 

 where x is the atmosphere of the body in question expressed in 

 units of the earth's known total atmosphere, and m is the mass 

 of the body expressed in units of the earth's mass. 



The mass of the moon being -- that of the earth, we get 



80 V 80 



7'5'54i6. 



discarding fractions, the moon's atmo- 



sphere as -?- that of the earth. But the diameter of the moon 



being to that of the earth as 0-264 to unit)-, the lunar surface 

 will "be to that of the esrth as 0-264- or 0-0697 'o I, and the 

 lunar atmosphere will be concentrated accordingly, bringing the 

 njean atmospheric pressure on the lunar surface to 



I 



I I 



7 1 c ^^ , or — . , 



-^ 498355 50' 



o 0697 



nearly of that of the earth's mean atmospheric pressure. Such 

 an atmosphere would support a column of mercury six-lenths of 

 an inch in height. Mr. Browning will recognise this as about 

 equal to the best vacuum obtainable in an old-fashioned air-pump 

 of average defectiveness. 



Such is the theoretical pressure upon every part of the moon's 

 surface, supposing the form of the moon to be a perfect spheroid 

 of rotation wilh a perfectly smooth surface. But the moon is 

 no such regular body. It presents far more irregularities in 

 proportion to its size than would our earth if the ocean were 



evaporated, and its depths laid hare so that our mountain heights 

 should be measured from the ocean bottom. Under such con- 

 ditions the bulk of even our atmosphere would occupy the ocean 

 valleys, and very rare indeed would be the remainder that 

 reached the mountain tops and elevated ridges of the earth. On 

 the moon wuh its filmy atmo-phere of imly six-tenths of an inch 

 mean pressure, the rarefaclinn on the high lands and mountains 

 would be carried beyond the lii.its of observable refractive power 

 under the conditions assumed — viz., of a special atmosphere 

 merging gradua ly into the universal interstellar medium. 



The visible edge of the moon which effec's the occultation of 

 a star must in almost eveiy possible case be formed by the ridges 

 and summits of the lunar mounta'ns, in no case by the bottom 

 of the lower valleys, for in looking horizontally across the moon's 

 rotundity these valleys and even the maiia must be foreshortened, 

 and their lower depths walled out of the reach of our vi>ion by 

 the great lunar elevations. Thus the occultation of a star would 

 occur without its previous plunging behind any outlying lunar 

 atmospheric mat!er of appreciable density. We must not forget 

 that Sir J. Ilerschel's calculation, which assigns one second of 

 refraction to an atmosphere equal to y.jVu of the density of the 

 earth, is based on the theory of a limited atmosphere with a 

 sharp and definite boundary suddenly terminating in a vacuum. 



But this rarefaction on the elevated portions of the moon 

 demands a compensating condensation or concentration of 

 atmospheric matter in the valleys, crater-pits, and maria. Here 

 the pressure on the moon's surface should considerably exceed 

 the calculated mean. This consideration suggests a very inte- 

 resting question. Would such an atmosphere, say capable of 

 supporting one inch of mercury, produce any observable effects? 

 If I am right in regarding water as one of the constituents of 

 the universal atmosphere, there are good reasons for supposing 

 that it would. 



The small share of water due to the moon would .all be raised 

 far above its low boiling point, early in the lunar day, by the heated 

 lunar surface. There would be no sea, no clouds, no rain, no 

 snow, but on the plains and ia the valleys a formation of hoar- 

 frost should occur at the lunar eventide, beginning just where the 

 sun's rays become too oblique to maintain the temperature of the 

 rapidly radiating lunar surface above the freezing-point. 



In a note appended to Mr. Lockyer's translation of M. Guil- 

 lemin's work on "The Heavens," the Rev. T. W. Webb 

 thus corrects the author's rather positive statements concerning 

 the total absence of a lunir atmosphere: "After all fair de- 

 ductions on the score of imperfection of observ-ation or precipi- 

 tancy of inference, there are still residuary phenomena, such as, 

 for instance, the extraordinary profusion of brilliant points which 

 on rare occasions diversify the Mare Crisium, so difficult of inter- 

 pretation, that we may judge it wisest to avoid too positive an 

 opinion." Now the Mare Crisium is a great depression of the 

 lunar surface close upon that edge of the moon which, to our 

 vision, first receives and loses the solar illumination. If I am 

 right, aqueous vapour should be suddenly forming there during 

 the early crescent period after the new moon, and the hoar-frost 

 should be as suddenly precipitated as this wide depression rolls 

 towards the d.arkness after the full moon. In that chapter of the 

 "Fuel of the Sun" which is devoted to the meteorology of the moon 

 and Mercury, I have discussed some of the theoretical results of 

 these conditions and the appearances they should present. I may 

 here merely add that, as the temperature of any part of the moon's 

 unmantled surface must directly and very rapidly vary with the 

 incidence of solar radiation, all the undulating regions of the 

 moon must at morning and evening have a very patchy tempe- 

 rature, the slopes towards the sun being hotter than our tropics, 

 while the opposite side of the same hill receiving the solar rays 

 with great obliquity, and radiating into space almost without 

 impediment, must retain a freezing temperature, and thus the 

 cryophorous phenomena, which Sir John Herschel describes as a 

 possible result of the contrasted temperatures of the opposite sides 

 of the moon, should be effected even by the shady lunar craters 

 and contrasted hill-slopes. 



On the highlands of the moon no apprecable amount of 

 hoar-frost precipitation should take place on account of the ab- 

 sence of sufficient atmos|)here ; but on the deeper maria, wher- 

 ever the conditions are the most favourable, the patchy tempera- 

 ture should produce patches of such precipitation. If any- 

 where visible, these should be seen on the Mare Crisium, on 

 account of its proximity to the edge of the moon, for there the 

 morning rays that strike most obliquely upon the cold slopes 

 would be the most effectively reflected towards the earth. Not 



