[Footnote 4:  In 1890 at Mount Wilson, California, Mr. W.H.  Pickering’s photographs of Mars on April 9th showed the southern polar cap of moderate dimensions, but with a large dim adjacent area.  Twenty-four hours later a corresponding plate showed this same area brilliantly white; the result apparently of a great Martian snowfall.  In 1882 the same observer witnessed the steady disappearance of 1,600,000 square miles of the southern snow-cap, an area nearly one-third of that hemisphere of the planet.]

The rapid and comparatively early disappearance of the white covering is, very reasonably, supposed to prove that it is of small thickness, corresponding perhaps to about a foot or two of snow in north-temperate America and Europe, and that by the increasing amount of sun-heat it is converted, partly into liquid and partly into vapour.  Coincident with this disappearance and as a presumed result of the water (or other liquid) producing inundations, the bluish-green tinge which appears on the previously dark portion of the surface is supposed to be due to a rapid growth of vegetation.

But the evidence on this point does not seem to be clear or harmonious, for in the four coloured plates showing the planet’s surface at successive Martian dates from December 30th to February 21st, not only is a considerable extent of the south temperate zone shown to change rapidly from bluish-green to chocolate-brown and then again to bluish-green, but the portions furthest from the supposed fertilising overflow are permanently green, as are also considerable portions in the opposite or northern hemisphere, which one would think would then be completely dried up.

No Hills upon Mars.

The special point to which I here wish to call attention is this.  Mr. Lowell’s main contention is, that the surface of Mars is wonderfully smooth and level.  Not only are there no mountains, but there are no hills or valleys or plateaux.  This assumption is absolutely essential to support the other great assumption, that the wonderful network of perfectly straight lines over nearly the whole surface of the planet are irrigation canals.  It is not alleged that irregularities or undulations of a few hundreds or even one or two thousands of feet could possibly be detected, while certainly all we know of planetary formation or structure point strongly towards some inequalities of surface.  Mr. Lowell admits that the dark portions of the surface, when examined on the terminator (the margin of the illuminated portion), do look like hollows and may be the beds of dried-up seas; yet the supposed canals run across these old sea-beds in perfect straight lines just as they do across the many thousand miles of what are admitted to be deserts—­which he describes in these forcible terms:  “Pitiless as our deserts are, they are but faint forecasts of the state of things existent on Mars at the present time.”