420 



KNOWLEDGE 



[Hov. 124, 1883. 



Friiiav till Sunday. Strong earth currents disturbed the 

 action of the telegraph and telephone lines. In Canada 

 and the United States, as far wtst as I't&li, the electrical 

 disturliance was felt Electricians say that the disturbance 

 was unlike any heretofore known, acting- on the wires in 

 strong waves, which produced constant changes in the 

 polarity of tlio current. A magnificent aurora was visible 

 throughout Canada and the United States. 



The phenomena were akin in many respects to those 

 desiriUil in the editor's treatise on The Sun, as accompany- 

 ing and following tlje great solar disturbance of September, 



SATURN'S RINGS. 



MR, LYNN explains that his remark in tho AlheuouiK 

 referred, not, as I supposed, to the passage in which 

 I thanked Captain Noble for calling attention to this 

 subject in Kn"ovvleui;e, but to my remark in KsowLEDtiK 

 for Oct 13 to the effect that a writer in the Athenauin 

 liad " followed Captain Noble's lead " — a remark founded, 

 as he correctly supposes, upon an article in the Atheiiatim 

 for Oct 7th, (juotiiig a letter of Mr. Lynn's as its autho- 

 rity. He adds : " Perhaps you will kindly e.xplain how 

 ascribing the lead to Captain Noble, whose letter, as Mr. 

 Maunder informed you, was written after mine, diflers 

 from ascribing the first notice to him." Since I referred 

 to the writer in the Mhfnitum, not to Mr. Lynn, as 

 following Captain Noble's lead, there is nothing, I appre- 

 hend, to explain. 



I may take this opportunity of noting that I attach the 

 least j>ossible importance to the question, Who first saw 

 with the telescope niiy feature which the telescope was 

 bound, earlier or later, to reveal. There is something to 

 my mind altogether inconsistent with the dignity of science, 

 in the childish logogriphs under which men really great in 

 science concealed their discoveries, lest theyshould lose claim 

 to the distinction of having been the first to see something. 

 Tliere are many, however, who speak of a first view of some 

 astronomical object as if it des<Tved no less credit than 

 the discovery of Neptune or the labours of tho Herschcls 

 among stars and nebulae R. A. PiiocTOH. 



HAS THE MOON AN ATMOSPnERE ? 



Bv W. Mattibu Williams. 



'PHE letter of Mr. B. J. Hopkins, p. .379, reminded me 

 I '.f Mr. Rinyard's paper on this subject, which I had 

 tiiarke<l In 1^ read when it appeared. 



It appears that he and otliers investigating this subject 

 have omitti-d the connideration of a condition which I 

 j')int'-d out in Chapter XV. of "The Fuel of the Sun," as 

 f'.llowH " In the determination of the pressure of the 

 n'riio'.plif^ro upon the earth w.; take our barometers to the 

 w-a l.v.l, i'..-., the lowest f»ortion of the earth's general 

 surface. We must in like manner take our itnaginary 

 liarometers to the lowest <j'.nrrnl surface of the moon tr) 

 <>t>tain this calculat<^ column of mercury of six tenths of 

 an inch ^entimatcd on the aiuutnption that the moon has 

 it« eravitation-e<|uivalent of general atmospheric matter). 

 Fro: , t!,.- . v,,..i,ive irregularity of the lunar surface it 



■ tficult to define rlearly where this lowest 



It murt \f at the Ijottom of the avcrrnge 



■ y» of the lunar surface, probably about the 

 i..':ii* ;•:-.! ; of the Orrnnu* I'rftcrUanirn, and the other dark 

 r«^on« calli-d the ' Maria ' by selenographists. 



" The observations of occulta tions are generally stated to 

 indicate no atmosphere, though some doubts as to tho 

 absolute truth of this have been expressed. Now, the edge 

 of the lunar disc, by which tho boundary of the moon's 

 diameter is measured, and which determines tho beginning 

 and end of an occultation, is formed by the summits of the 

 lunar peaks and ranges, levi'lled by the eU'eot of perspective; 

 for we look upon them horizontivlly, as though from their 

 own summit level. The only portion of the lunar atmo- 

 sphere remaining for tho purposes of horizontal refraction, 

 visible from the earth, must be tliat which rises above these 

 lofty summits.'' 



The passage that confirms my supposition of tho omission 

 of this consideration is that commencing Mr. Kanyard's 

 last paragraph, on page L'lT), wliere ho says — " It will be 

 seen tliat the light which enters the slit close to the moon's 

 limb must, if there be a solar atmosphere, have passed 

 through the densest portions of it." 



We must remember that in comparing the rarefaction of 

 the lunar atmosphere on the mountains of the moon with 

 that on our terrestrial mountains, we have an ocean of 

 water filling the deepest depressions of the earth, and our 

 atmosphere starts from the surface level of this ocean, 

 while in the moon the corresponding depressions would 

 operate us atmospheric basins, into which an excessive pro- 

 portion of the general atmosphere would be compressed and 

 concentrat(!d. 



The diU'erence between the results obtained by M. 

 Thollon and JI. Trcpied may, I think, bo explained by the 

 possibility of tlu^ railial slit of M. Trepied catching tho rays 

 coming through a notch or depression between tho lunar 

 mountains, while the tangential slit of M. Thollon swept 

 across the summits and ridges. 



If I am right in this, the atmospheric lines seen in the 

 first case should Ije displayed of full length ; in the second 

 only as fragments, occurring where the length of the slit 

 visually crossed a notch caused by a lunar valley running 

 transversely to the line of the visible lunar horizon or limb. 

 This result would have tho puzzling and contradictory 

 character described. 



[We may conveniently consider what the difference 

 •would be in a valley, say two miles below the mean level 

 (though such valleys must Ik; too few and .small to form 

 any appreciable part of the moon's limb), and at a height 

 of two miles above that level (though parts at that height 

 can in like manner form but a very small part of tho 

 moon's apparent edge). On our earth i\w atmosphere 

 doubles in pressure for each three-and-a-half miles of 

 descent ; on the moon, where gravity is one-sixth, for each 



twenty-one miles, llenc 



1 is the ratio between the 



densities at lower and higher of these levels, we have the 

 relation : — 



4 

 log. (.= jjlog. 2 



(the logarithms being hyperbolic.) 

 Now, Neperian log. 2=0-0931472 

 4 



2I)2-' 



i5888 



i;}202«0=log. 114. 



Ilcnce the variation of density to which Mr. Williams 

 attaches so much invportance is in the ratio of 1 1 1 to 100, 

 or (very lurarly) of H to 7. All that we know of the con- 

 formation of tlie moon's surface, however, assures us that 

 nothing like a difl'erence of 4 miles could exist between 

 any appreciable parts of the moon's edge. A difference of 

 21 miles in level would bo required to produce a ratio of 



