Meanwhile the rifts themselves, from being lines which penetrate the cap from its periphery in toward its centre, end by traversing it in its entirety and separating portions which, becoming outlying subsidiary patches, themselves proceed to dwindle and eventually disappear. The rifts usually take their rise from such broader parts of the cap-encircling blue belt as make beads upon that cordon and are clearly spots where the product of the melting of the cap is either specially collected, or produces its most visible effect.
So far the description might apply with substantial accuracy to either cap. Yet the conduct of the two is in some ways diverse and begins to accentuate itself from this point on.
Northern Cap hooded with vapor.
From the time that the north polar cap reaches a diameter of about twenty-five degrees, a singular change steals over it. From having been up to then of a well-defined outline it now proceeds to grow hazy and indistinct all along its edge. This change in its character at the same period of its career has been quite noticeable at each of the three last oppositions, so that small doubt remains that the metamorphosis is a regularly recurrent one in the history of the cap. Coincident with the obliteration of its contour, its dimensions seemingly enlarge. It is as if a hood had been drawn over the cap of a dull white different from the dazzling brilliance of the cap itself and covering more ground. Such is probably what occurs; with vapor for veil. The excessive melting of the cap produces an extensive evaporation which then in part condenses to be deposited afresh, in part remains as a covering, shutting off from our view the outlines of the cap itself. It would seem that at this time the cap melts faster than the air can carry it off. A sort of steaming appears to be going on, taking place in situ. For it clearly is not wafted away. The time of its coming too is significant. For the season is May 15, the height of time for a spring haze to set in. Then later it dissipates with the same quiet indefiniteness with which it gathered.
Northern Cap unmasked.
It is some time in Martian June before the spring haze clears away, and when it does go, only a tiny polar cap stands revealed beneath it, from six to eight degrees across, or from a tenth to a fifteenth of what it was when it passed into its curious spring chrysalis. The date of emergence varies. In 1903 it occurred early, the haze not being marked after June 3, though recurring again at intervals for a day or so. In 1905 it was later; perceptibly thin after June 21 it did not certainly clear away till June 9 and came back again on July 16 and possibly on the 25th.
These vicissitudes of aspect give us glimpses into a sweet unreasonableness in Martian weather which makes it seem more akin to our own. And this on two counts, diurnal and annual. From day to day atmospheric conditions shift for purely local cause; while, furthermore, successive Martian years are not alike. In some the season is early; in others late. So that Mars is no more exempt than are we from the wantonness of weather.
Clearly disclosed thus reduced to its smallest possible terms it remains for some months of our days, for six weeks of its own. During that period it continues practically unchanged, neither increasing nor decreasing significantly in size, nor altering notably in aspect. Measures of the drawings of it then make it from five to eight degrees across and it is possible that it really fluctuates between narrow limits, though its clear-cut outline at all times renders the variation difficult to explain. We are not so near it as we could wish; for on these occasions even at their best it is over two hundred times as distant as the moon and the greatest magnification possible still leaves it a hundred thousand miles away.