THE MOON.

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and colder. The water which in ages past doubtless existedupon it (or at any rate nearly all ol' it) has long since vanished,having perhaps been absorbed into the rocks which form thelunar crust. In like manner the atmosphere, formerly belong-ing to it, which, owing to the Moon ’s comparatively small mass,was probably of moderate amount, has disappeared, or so nearlydisappeared as hardly to leave any certain traces of its existence.

Among the proofs which may be given for this last state-ment, the following may be mentioned. We every now andthen see the Moon pass between us and some of the myriadstars that stud the firmament. When this occurs, the dis-appearance of the star behind the lunar disc and its re-appear-ance are sharp and sudden, indicating that there is noappreciable effect produced by any lunar atmosphere. Again,we meet with no definite evidence of its existence when wewatch the progress of a solar eclipse.* If, as some think, weare occasionally able to detect signs of twilight upon theMoon , they are so exceedingly slight as to indicate that anyatmosphere upon it cannot possibly possess a density greaterthan the 200th part, ‘or it might perhaps be said the 500thpart, of the density of that upon the Earth ’s surface.

The contrast between sunlight and shade upon our satellitemust consequently be intense. The blackness and sharpnessof the mountain shadows may well form one of the mostremarkable features in a telescopic view of the Moon . If,however, we could transport ourselves thither, we may besure that the reality would surpass the utmost bounds of ourimagination; one step would take us from the deepest black-ness of night into the fullest blaze of the unclouded Sun.There would be no diffused atmospheric radiance, no softening-down and toning-off of shade. In some positions an oppositemountain might reflect a little light to an observer standingin the midst of a dark shadow, but such cases would beexceptional.

The sky would have none of that beautiful blueness which

* Since the above was in print, news has however reached us, thatsome traces of its presence have been detected with the spectroscope by theFrench observers of the total solar eclipse of May 17th, 1882, in Egypt .