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 continent and the colour of the ocean. Possibly a great mountain mass like the Alps would be sufficiently noticeable to permit some conjectures as to its character to be formed. But it is obvious that it would be hopeless to expect to see details. The smallest object that would be discernible on Mars must be as large as London. It would not be possible to see a point so small as either Liverpool or Manchester would be if they were on that planet. There is no doubt a remarkable contrast between the dark colours of certain parts of Mars and the ruddy colours of other parts. We would, however, be unwarranted in asserting that the former must be oceans of water, and the latter continents of land. This may indeed be the case, and most astronomers, I believe, think that it is the case, but it certainly has not yet been proved to be so.

Undoubtedly the most striking piece of evidence that can be adduced in favour of the supposition that there is water on Mars is derived from the "snowy" poles on the planet. The appearance of the poles on Mars with their white caps is one of the most curious features of the solar system. The resemblance to the structure of our own polar regions is extremely instructive. It is evident that there must be some white material which from time to time gathers in mighty volume round the north and south poles of the planet. It is also to be noticed that this accumulation is not permanent. The amount of it waxes and wanes in correspondence with the variations of the seasons on Mars. It increases at either pole of Mars during the winter of that pole. In this respect the white regions, whatever they may be composed of, present a noteworthy contrast to the majority of the