THE PLANET MAES.

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through which they are seen. The non-existence of any appre-ciable atmosphere would (as in the case of the Moon ) he opposedto the existence of water ; its presence confirms it. We alsofind that the hemisphere of the planet which is enjoyingsummer, in which the weather might he expected to be drierand less overcast, is much more clearly seen, as though it weremore free from aqueous vapours and cloud, than that which ispassing through the season of winter.

Moreover, if we test the light received from the planet bymeans of the spectroscope, and compare it with that receivedfrom the disc of the Moon, * w r hen the two are seen nearly sideby side; the former shows dark lines, representative of wateryvapour, which are wanting in the case of the latter.

We have, therefore, abundant indications of the existence ofwater upon Mars ; and, as there is no doubt that water wouldreflect much less light than land, we are amply justified inbelieving that the darker markings are caused by its presence.Their occasionally greenish tint, may atford some farther con-firmation of this opinion.

But, in addition to all the above reasons, there is one moreconvincing still. It has to do with certain brilliant white spotswhich involve, and surround to a certain distance, the Northand South Poles of the planet. As the summer progresses, ineither hemisphere, the spot belonging to the correspondingpole rapidly diminishes in size. On the other hand, as the winterprogresses, it undergoes a great increase and extension. It isalso noticed, that the variations of size in the southern spot arcgreater than those of the northern one (as is also the case uponthe Earth , see Lecture VIII., p. 190) ; a coincidence especiallyimportant when we remember that the summers of the southernhemispheres, both of Mars and of the Earth , occur iu thosehalves of their respective years in which they at present passthrough their nearest approaches to the Sun. We arc, more-over, able to watch the increase and diminution of the southernpolar spot with peculiar facility, because the south pole isthat which is inclined towards us whenever the proximity of

* Such a comparison was made by Dr. Huggins; see Monthly NoticesE. A. S. for March 1867 (vol. xxvii., p. 178).