NA TURE 



305 



THURSDAY, AUGUST 10, 1876 



THE MOON 

 The Moon and the Condition and Cotifigtirations of 

 its Surface. By Edmund Neison, F.R.A.S., &c. 

 (London : Longmans, Green, and Co., 1876.) 



FROM the earliest ages our satellite has attracted a 

 great portion of the attention of astronomers of all 

 nations, and from its proximity to us it is only right that 

 it should still have a large amount of astronomical labour 

 bestowed on it. This labour is divided into two kinds, 

 that of ascertaining its motions in space, and that of 

 inquiry into its physical constitution. It is of this latter 

 research or the study of selenography, chiefly, that the 

 present work treats. The author tells us that he has taken 

 the "Mond" of Beer and Madler as a basis, but that 

 the greater portion of the material has been mainly derived 

 from eight years constant selenographical observations, 

 principally made with a 6-inch equatorial of fine defini- 

 tion and with a 9J inch With-Browning reflector. Also, 

 use has been made of some hundred lunar sketches made 

 of late years by different astronomers, and which from 

 time to time have been sent to the author ; the work, 

 therefore, is as complete as our present knowledge enables 

 it to be. The first chapter of the book treats of the 

 motions, figures, and dimensions of the moon, and men- 

 tion is made of the elliptic inequality, discovered by 

 Hipparchus, evection, variation, and annual equation. 

 The alteration in appearance of the lunar surface, due to 

 libration, is a matter of the utmost importance in seleno- 

 graphy, and the discussion of its effects, together with the 

 formula for computing the same at the end of the book, 

 will be useful to those interested in lunar observations. 

 The author then proceeds to discuss the question of a 

 lunar atmosphere, and his arguments in favour of the 

 same, having a surface-density of ^\^ that of our air, are 

 extremely forcible. It has always seemed strange that the 

 moon should have had neither air nor water, or almost 

 none, and we are glad to see that it is not incompatible 

 with appearances that a mass of air and water should 

 have existed comparable to ours, when the relative mass 

 of the moon and earth are considered. 



The weather-beaten and ruined portions of the moon's 

 surface are referred to as indications of the effects of these 

 agents ; and in favour of the existence of an atmo- 

 sphere, Mr. Neison points out that "it may be reasonably 

 supposed that the ratio of the mass of the primitive lunar 

 atmosphere to the mass of the moon would be a similar 

 ratio to that which obtains on the earth, considering the 

 close connection between the two ; but such are the con- 

 ditions prevailing on the surface of the moon, that so far 

 from the resulting atmosphere resembling in surface- 

 density that of the earth, it would only be ^V as dense, 

 for not only is the surface of the moon as compared with 

 its mass much greater, but the force of gravity at its 

 surface is much less powerful, so that from these causes 

 the atmosphere would occupy a much greater compara- 

 tive volume, and consequently possess a very small 

 density." One would, at first sight, fancy that -^^ was too 

 small a probable estimate, but when we consider that the 

 mass of the moon is -^^ that of the earth and its surface 

 Vol. XIV. — No. 354 



iV> we get, per unit of area, W of the mass of the terres- 

 trial atmosphere. 



The force of gravity on the moon's surface being about 

 ^ of that on the surface of the earth, the pressure of the 

 atmosphere at the surface will be about ^^ of the pressure 

 of our atmosphere ; correcting this for the probable effects 

 of temperature, we get somewhere about ^V ^s the 

 surface density. The author explains the absence of 

 water and the disappearance of a great portion of the 

 above small quantity of atmosphere by drawing a parallel 

 between the surface of the moon and earth, and stating 

 that " the joint effect of the action of the terrestrial surface 

 oceans and atmosphere has been to form the present 

 crust of the earth, where is to be found locked up an 

 immense mass of water and of the constituents of our 

 atmosphere which originally formed part of the early 

 terrestrial oceans and atmosphere, and by this means 

 probably a very considerable portion of these must have 

 been by now removed. A similar action would have 

 ensued on the moon with this important difference, that 

 as, relatively to these masses the lunar surface is more 

 than six times as great as the earth's, this absorption of 

 the oceans and of the atmosphere would have been not 

 only more rapid, but have been carried to six times the 

 same extent under the same conditions." The present 

 surface density, he therefore argues, may be now \ of its 

 original state, or about -^^ of the density of that of the 

 earth. 



The estimated density of Bessel and others from re- 

 fraction, of about loVa of ^^^t on the earth, is referred to, 

 coupled with a remark that the temperature was assumed 

 by him to be uniform and a factor depending on the dif- 

 ference of the form of gravity at the surface of the earth 

 and moon omitted, and if correction is made for these, 

 the result should be uItt as the surface density. From 

 observation of occultations it has long been known 

 that a difference of some 2 " existed between the semi- 

 diameter of the moon as determined by occultations 

 and that determined by direct measurement ; irradiation 

 accounts for a part of this, leaving the rest to be accounted 

 for by horizontal refraction, and this, we read, renders a 

 surface density of ^^ of that of the earth's atmosphere 

 possible, but from other considerations the author puts the 

 probable density at jJq. The effect of such an atmo- 

 sphere in mitigating the climate is shown, but it is not 

 quite easy to see from the present evidence that such an 

 atmosphere, having a pressure of about one terrestrial 

 ounce to a square inch, will account for lunar appearances, 

 or even if we take the pressure at one time to have been 

 ^V of that on the earth, or six ounces. This will require 

 future observations to settle. We are inclined to think 

 that the author is rather over-zealous in his cause when 

 he states to the effect that the mass of atmosphere over a 

 square mile in area must be estimated in millions of tons. 

 This can scarcely be the case, judging from the above- 

 estimated pressure. The occultations *of stars, the blue 

 halo occasionally seen around isolated craters, and Lord 

 Rosse's experiments upon radiation from the surface of 

 the moon are all discussed. The author, however, 

 candidly acknowledges that no definite results can be 

 obtained from them either one way or the other, but is 

 convinced that the balance of evidence is in favour of an 

 atmosphere of considerable magnitude, although of slight 



Q 



