154 



KNOWLEDGE. 



[August 1, 1898. 



The intensity of the solav heat received by Mars is, we 

 know, only about two-fifths of that received by the earth ; 

 but if the earth only received two-fifths of the heat which it 

 does now, the temperature would be so low that the seas 

 would be completely frozen. Mars is probably consider- 

 ably colder than the earth, especially at night, but it does 

 not appear to be as cold as the distance of Mars would lead 

 us to expect. There must, then, be some cause in the cir- 

 cumstances of the planet itself, preventing it from getting 

 as cold as the earth would under similar circumstances. 



The only causes of heat that appear to be possible are 

 ('() internal heat, and (/-) a pecuhar constitution of the 

 atmosphere. It does not seem likely that the internal 

 heat of Mars can exceed {or even equal) that of the earth, 

 so that we are almost com|<elled to conclude that the 

 atmosphere is the cause. We know that a superabundance 

 of aqueous vapour in the atmosphere would produce the 

 eti'ect ; and we cannot suggest any other cause whicii 

 appears reasonably probable. But if it be due to aqueous 

 vapour, the atmosphere must be much damper than ours, 

 because ours would not produce the eii'ect, or anything 

 like it. Yours faithfully, 



Dublin. .J. K. Holt. 



[The precipitation of aqueous vapour into cloud in the 

 earth's atmosphere is evidently dependent on other con- 

 ditions besides temperature, or clouds would form and 

 spread over the heavens every evening as the sun sets. 

 Recent investigations seem to show that the dust-laden 

 condition of our atmosphere is intimately connected with 

 the formation of clouds, and it seems probable from (/ priori 

 considerations that the atmosphere of Mars is less dust- 

 laden than the atmosphere of our earth. Other conditions 

 remaining unchanged, the amount of dust in the atmosphere 

 is dependent on the velocity of the wind, and the feeble 

 gravity at the Martial surface would, as Mr. Maunder 

 pointed out in his article on " The Climate of Mars," 

 published in Kxowlbdi;e for September, 1892, lead us to 

 conclude that the Martial winds blow more feebly than 

 terrestrial winds, for all winds have their origin in rising 

 currents of heated air. If the atmosphere of Mars were 

 as dense and vapour-laden as our own, we should expect 

 to see sunset tints on the limb of Mars when gibbous ; 

 but no such tints are traceable, and we can hardly expect 

 that the atmosphere of Jlars is damper than our own, for 

 as Mr. Holt correctly points out, the amount of aqueous 

 vapour in the Martial atmosphere must depend upon the 

 temperature of the Martial surface, and not on the density 

 of the Martial atmosphere. I am therefore inclined to 

 concluda that the surface of Mars is much colder than the 

 earth's surface, and that the white material which is 

 deposited at the Martial poles, when turned away from the 

 sun, and which melts or disappears as the poles turn 

 towards the sun, cannot be snow, but is more probably some 

 vapour which is precipitated as snow at a far lower tem- 

 perature than 82' Fahr. Carbonic acid gas when frozen 

 forms a very white, snow-like substance, and an atmosphere 

 similar to our own, or any other mixture of gases forming 

 an atmosphere about the planet, would, if solidified into 

 small crystals, appear white and snow-Uke. — A. C. Ranyard.] 



To the Editor of Knowledge. 



Df.ar Sir, — The question raised by Mr. Holt in your 

 last number is well worthy of consideration. The Greek 

 word corresponding to the Latin "focus " is hrria, but as that 

 word has never, I believe, been applied in a mathematical 

 or astronomical sense, I fear " pericstia " would hardly 

 be an appropriate word to express the point in the orbit of 

 a satellite which is nearest to its primary. The objection 



to " pericentron " is that its etymological meaning would be 

 ambiguous. If I might venture on a small pleasantry (to 

 be taken in a courteous sense), I would kick against that 

 suggestion, as reminding one of a (ireek proverb about an 

 ox-goad. 



Independently of "focus" being a Latin word, in the 

 mathematical sense every ellipse has two foci, one towards 

 each apse ; so that if there were a Greek word exactly 

 corresponding, the prefixing "peri" to it would not make 

 it clear whether the point nearest to or farthest from the 

 primary planet were intended. Now for these two points 

 collectively we have already the word " apsides '' ; and if it 

 be desired to coin a word to distinguish that whicli is 

 nearest the centre of force, perhaps " protapsis " might do as 

 the principal or most important apse, whilst " hyperapsis " 

 (upper apse) would do for the other — the two corresponding 

 to perihelion and aphelion in the orbit of a primary planet. 

 As Montucla points out, the word " focus" was introduced 

 into the conic sections by modern mathematicians from 

 its optical signification ; the ancient geometers designated 

 the point by the Latin expression, jniiictiim cowjiaratinnis, 

 and its Greek equivalent. 



Mr. Holt alludes to the lii/hriil word " perijove," which 

 has obtained some currency and is decidedly objectionable, 

 though probably the correct form could never be adopted. 

 I'nfonunately, there are many instances of want of sutlicieut 

 care in the formation of words from the Greek. In a 

 conversation I had recently with Prof. Barnard on the fifth 

 satellite of Jupiter, he informed me that many persons in 

 California (with whom he did not himself agree) wished 

 him to name it Eureka, the word being on the seal of that 

 state. There are obs'ious objections to this application of 

 the word ; but it is unfortunate that it has been adopted 

 into English in an erroneous form, for it certainly should 

 have an aspirate and be spelt Heureka, as doubtless 

 pronounced by Archimedes when fresh from his bath. 

 IMackheath. Yours faithfully, 



•Tuly isth, lHi)3. W. T. Lvxn. 



»-•-* 



THE SPECTRA AND PROPKR MOTIOXS OF STARS. 

 To the Editor of Knowledge. 



Sir, — As I am jugt leaving on a vacation trip, during 

 which I shall be away from books and documents, 1 must 

 defer for the present any remarks on the letters of your 

 correspondents Prof. Kapteyn and Mr. Iloia^ton. We 

 undoubtedly require more research before ariiving at 

 definite results, and the extension of the Draper Catalogue 

 to the southern hemisphere, when completed, will furnish 

 very important materials. I may mention that, suspecting 

 that many faint stars with considerable proper motion 

 described as Sirian in the Draper Catalogue were really 

 solar, I wrote to Prof. Pickering, who kindly directed a 

 re-examination of the spectra of the stars mentioned in my 

 letter, and has lately sent me the result. A very con- 

 siderable proportion of these stars turn out to be really 

 solar stars, mainly of the Capellan type. I made some 

 furthe)- inquiries with respect to Capellan stars whose 

 proper motion appeared to be smaller than I expected, but 

 in these instances the new examination led, in the great 

 majority of cases, to the same result as the old. The 

 principal exception is Rigel or /3 Orionis, whose spectrum 

 is not F but B — this spectrum, according to my view, 

 denoting the most brilliant, and therefore the most 

 distant (relatively to their magnitudes), of all stars. The 

 spectrum of IHiSO Groombridge is G. 



Truly yours, 



10, Earlsfort Terrace, Dublin, W. II. S. Moxck. 



July 5th. 



