174 



KNOWLEDGE. 



[September 1, 1898. 



G passes through. Of this, a (1 -a)G is absorbed in the 

 second four miles, and (1— rt)-G passes through. Pro- 

 ceeding in this way, the heat which finally escapes fi-om 

 the fifth four-mile layer is (l-n)^G, and therefore the 

 total heat absorbed in the atmosphere is a [5 — \0a + \Qii- 

 -5rtS + a^)G. 



We do not know what a is exactly ; however, we may be 

 pretty sure that it represents a fraction which is not very 

 large nor yet infinitesimally small. Consequently, we 

 may take the above as probably between 4«G and 2((G. 



We know that G is about -411. 



So, if the heat conserved on the earth be wH, that con- 

 served on Mars is probably between 6"H and -S^/H. 



This would account for a surface temperature not very 

 difierent from our own, but probably a little u-anner. 

 Since Mars is probably cddi-r than the earth, the con- 

 clusion I draw is that the atmosphere is somewhat drier 

 than ours. 



Of coarse the figures I took, four miles and twenty miles, 

 are imcertain. But the general argument does not involve 

 the exact figures ; the gist of it is that in estimating the 

 effect of the atmosphere of Mars on the temperature, the 

 great thukni-ss of the absorptive stratum must be taken 

 into account, and wUl cause the atmosphere to conserve 

 much more of the incident heat than ours does. 



The same line of reasoning applied to the planet Venus 

 would make it excessively hot. 



J. R. Holt. 



[I agree with Mr. Holt that the density of the aqueous 

 vapom- present in the lower strata of the atmosphere of 

 Mars will depend upon the temperature of the Martial 

 surface, and not upon the density of the Martial atmos- 

 phere, for at any given temperature as much aqueous 

 vapour rises into a vacuum as into a dense and dry 

 atmosphere. With gravity at the surface of Mars equal to 

 only 39 per cent, of gravity at the earth's surface, the density | 

 of the Martial atmosphere will be halved at a height about 

 two and a half times as great above the surface of Mars 

 as the height above the earth's surface at which the 

 density of the earth's atmosphere is halved ; consequently, 

 an atmosphere of aqueous vapour of any given density at 

 the surface of Mars will, if it is not precipitated into cloud 

 or ice particles, extend to about two and a half times (not 

 five times) as great a height above the surface of Mars as 

 it would extend above the surface of the earth, ■ and any 

 absorption due to aqueous vapour would correspond to the 

 absorption of a layer of similar density two and a half 

 times as thick above the earth's sm-face. But if I under- 

 stand rightly. Prof. Langley's bolometer observations have 

 thrown great doubt upon the theory that aqueous vapour 

 transmits solar heat while it absorbs the non-luminous 

 heat of long wave-length radiated by the earth's surface ; 

 and it will be remarked that if Mr. Holts theory held 

 good, the moon's surface, with lunar gravity equal to only 

 about a sixth of terrestrial gravity, ought to become in 

 the course of the limar day much hotter than the earth's 

 surface, but if the observations of Profs. Langley, Very, 

 and Boys are to be relied upon, this is not the case. — 



A. C. Rany.\kd.] 



— I » I — 



To the Editor of Knowledge. 



S)K, Would not the establishment of a " Scientific 



Review of Reviews," published monthly, and including 

 summaries of the transactions of the learned societies, be 



the best method of solving the problem you refer to in the 

 August number of Knowledge (p. 148) "? A mere index, 

 without some sort of indication as to the drift and bearings 

 of the article in question, would hardly lessen the difficulty 

 very much. Such a review would have to be edited by 

 one of those you term scientific " scholars," assisted by a 

 strong staff" ot specialists and correspondents. The neces- 

 sity of keeping the review within reasonable limits would 

 prevent its injuring the already existing magazines by 

 doing more than just summarize their articles. So many 

 of our greatest feats in science have been achieved by 

 private individuals that it would be a great pity to intrust 

 so important an undertaking to any department of the 

 Government. Yours faithfully, 



Malvern. H. St. A. Aldek. 



[Such a review would certainly not pay commercially. 

 The work would probably be most satisfactorily done if 

 the various learned societies could induce specialists to 

 contribute an index of papers in their own special depart- 

 ments. Prefix of scientific papers, similar to the head notes 

 of cases published in Law Reports, would be most difficult 

 to prepare. — A. C R.] 



• It follows from this tliat about two and a half times as much 

 aqueous vapour would, for any given temperature, be evaporated from 

 the surface of Mars as from an eiiual area of the earth's surface. 



To tlie Editor of Knowledge. 



De.\k Sik, — If I might be permitted to make a small 

 criticism upon Dr. Anderson's interesting article in the 

 July number of Knowledge, it would be with reference to 

 the sentence : — " It is perhaps too late now to restore to 

 9 its ancient style and title of Achernar, but a at least 

 sb,)uld be made to yield up its usurped honours." But 

 a. Eridani is really " the last star m the river " as we know 

 the constellation, so that one hardly sees why it should be 

 deprived of the name Achernar, whiih has that meaning, 

 because Ptolemy called another star (our fl Eridani) "the 

 last in the river" ; it being the last in the stream which 

 was visible in his latitude. The proper thing, it appears 

 to me, would be to retain Achernar as the name of 

 a Eridani, and to call 9 Escbatos or Ptolemy's Eschatos. 



It is certainly very remarkable that in his time (and for 

 centuries afterwards) there should have been a star of the 

 first magnitude at the extreme end of Eridanus as visible 

 in Alexandria, and that there should also be a star of the 

 first magnitude at the extreme end of that river as extended 

 by observations made further to the south in more recent 

 times, whilst Ptolemy's first should have faded down to the 

 third magnitude. But there are more strange things in 

 heaven and earth than our philosophy dreams of, and 

 there seems no escape from the conclusion. The nature of 

 the variability, then, of 6 Eridani would seem to be almost 

 unique ; for it is not a case of gradual diminution through 

 the centuries. Ptolemy, Al Sufi and Ulugh Begh found it 

 of the first magnitude ; Halley, about two centuries after 

 the latter, observed it of the third magnitude, at which it 

 has remained ever since. Gould, in the Uninomvtria 

 Anicntinii, registers 2*6 as the precise magnitude when he 

 formed his catalogue. 



But there is a further difficulty connected with it, which 

 is this. Bayer, in the tables accompanying his constellation 

 maps (pubhshed in 1003) assigns to 6 Eridani its modern 

 magnitude, the third. How did he obtain his knowledge 

 of this, which carries back the change of laagnitude to 

 nearly a century before the time of Halley P Tycho did 

 not obseive it, for the best of all possible reasons ; and it 

 would almost seem as if Bayer took more trouble about 

 some of his data than has generally been supposed. His 

 a. Eridani (marked of the first magnitude) is undoubtedly 

 that in the far south which is now commonly called 



