192 



KNOWLEDGE 



[October 1, 1891. 



a sustained uniformity in the dischai'ge of vapour over i 

 such lonj; lines seems to me iosnrmountable. 



If the rays were occasionally interrupted by blanks, or if 

 we could, even in a few cases, detect the fissure, this i 

 solution would appear more likely, inasmuch as the visible i 

 " clefts ■' are so obviously fissures in the crust, whence, 

 here and there along their course, vapour has been given 

 off which has been piled as snow cones, or little rings, 

 called craterlets. 



But there is no evidence of a fissure, as far as I am 

 aware, in any of the rays, nor can we trace any 

 evidence of a piling up, as in the strings of craterlets ; the 

 ray is simply a whitish streamer destitute of all structure, 

 and which nowhere deforms the detail or modelling. 



It is conceivable that among 1000 rays owe might be due 

 to a fairly straight (and continuous) minute fissure, whence 

 aqueous vapour had been so ii/unlih/ iwludcd nil alumj as to 

 give the appearance of a w^hite streamer, as the snow was 

 deposited on each side. That two such cases should occur 

 would, hoTvever, be remarkable, and that "// should present 

 such a combination of features seems to me really in- 

 credible. 



If we compare, in plan, the lines of fissure passing out 

 from our great terrestial volcanoes (such as those figured 

 on page l\'l of Judd's " Volcanoes "'), with the hmar ray- 

 systems, we at once see a radical difference. In our case 

 the lines are not only strongly curved and branched, like 

 rivers, but occasionally fork at right angles and terminate 

 generally in hranrhleU, like a tree top, a feature which is 

 <iuite foreign to the rays, as far as I am aware. 



The rays, as a rule, appear to be solitary tracks, and 

 this seems to me to preclude the idea of their being due 

 to fissures, the more so as the visible "clefts" on the 

 moon are often branched, forked, or crossed. 



So that neither faults, trap-dykes, or fissures, exuding 

 aqueous vapour, would seem to solve this perennial 

 enigma. 



Sibsagar, Assam, India. S. E. Peal. 



Auijust -ISth, 1891. 



[I do not think, with Mr. Peal, that the " clefts are so 

 obviously fissures in the crust." Mr. Neison was inclined 

 to regard them as the dry beds of lunar watercourses or 

 rivers. At page I'l of his book on " The Moon," Mr. 

 Neison says : " With regard to the true nature of these rills 

 or clefts absolutely nothing is known, whilst they are too 

 delicate objects to allow much, if any, of the details of 

 their formation to be made out. It has been supposed 

 they are cracks or fractures in the lunar surface, but their 

 intersection and general conditions of existence seem quite 

 inconsistent with such a supposition, more especially in 

 their behaviour with reference to the various formations 

 they pass through, round or over. In many points they 

 bear some resemblance to the dried beds of lunar water- 

 courses or rivers, but in many features do not seem in 

 accord with such an origin, though perhaps it presents the 

 most feasible explanation of their nature of all." We should 

 not see such cracks in the lunar crust unless they were at 

 least a quarter of a mile broad, and it does not seem to me 

 probable that the precipitous sides of such cracks would 

 stand (even under the action of lunar gravity) if the cracks 

 were many miles in depth. The sides would fall together 

 imless they were supported by pressure from the opposite 

 precipice at very frequent intervals. One can conceive 

 of such a deep crack filled with di'brix, or of a fault many 

 hundred miles in length, if the opposite sides pi-essed 

 against one another at sufficiently frequent intervals to 

 give support. It is clear that the lunar rays have some 

 connection w'ith the volcanoes from which they radiate, 

 and the theory which seems to me most probable is that 



they correspond to radiating cracks or faults which would 

 not be seen from the earth. 



Into such a fault or crack the lunar atmosphere would 

 enter, however rare it may be at the surface, and it would 

 be considerably compressed at great depths. We know- 

 that on the earth the atmosphere sinks into the interstices 

 of the soil, and that on every fall in barometric pressure 

 it rushes forth again, laden in many mines with gas. 

 Everyone is familiar with the earthy smeU of the air after 

 a shower of rain, when the lowered pressure of the baro- 

 meter allows the air which has been forced into the soil to 

 rush back to the surlace. No doubt the lunar atmo- 

 sphere is similarly compressed into the soil, during the 

 com'se of every lunar day, owing to the increased 

 weight of the lunar atmosphere caused by the evaporation 

 into it of aqueous vapour from the snows exposed to the 

 direct heat of the sun. The air would be forced more 

 freely into such deep fissures and cracks than into the 

 ordinary soil, and it would return to the surface during 

 the lunar night. We may leel sure that the expansion 

 of the air on reaching the surlace would cool it, as the air 

 is here cooled by expansion as it blows up a mountain 

 side. This is well known to be the cause of the gi'eat 

 rain-fall on mountain tops, for the air, when cooled, will 

 not carry as much aqueous vapour as at a warmer 

 temperature. Similarly the lunar air would be cooled by 

 expansion as it reached the surface, and it would deposit 

 its moisture in the neighbourhood of the vent. 



It does not seem to be altogether improbable that air 

 should be pretty equally exhaled along the course of a long 

 fault or fissure. The branching structure figured in 

 Prof. -ludd's book on volcanoes, referred to by Mr. Peal, 

 does not represent cracks, but " intrusive masses of 

 dolerite." — A. C. Kanyaed.] 



THE ABSEXCE OF A LUXAR ATM081'HEEE. 

 I'o the Editor of Knowledge. 



Dear Sir, — While composing a lecture on Astronomy, a 

 few months ago, the following idea occurred to me as a 

 possible explanation of the circumstance that our satellite 

 is destitute of atmosphere. 



It is well known that when comets approach the sun, 

 large quantities of the gaseous portion of their contents 

 are repelled by some powerful influence, probably the 

 electrical action of the sun. 



If, then, we assume that the earth has, at one time, 

 passed through a sun-like stage, does it not seem probable 

 that it would then exert a similar influence upon the 

 atmosphere of the moon, the result of which would finally 

 be that our satellite would be entirely denuded of her 

 atmosphere '.' 



If this explanation be the correct one, we may expect to 

 find that all the other satellites in our system are likewise 

 destitute of atmosphere, owing to this action on the part 

 of their primaries. 



The circumstance, accordingly, that no trace of an 

 atmosphere has ever been discovered in connection with 

 the satellites of .Jupiter, furnishes confirmation of the 

 above-mentioned theoiy. 



Alexander C. Henderson. 



Mount Pleasant Manse, 



Newburgh, Fife, Scotland, 



9(/i September, 1891. 



We have no evidence that gaseous matter is repelled 

 from the heads of comets and driven into space. The 

 polarized condition of the light derived from the tails of 

 comets shows that the light of the tail is principally 



