As we saw in the annual history of the polar caps a dimness somewhat different affects the northern cap in May and June. After the melting of the cap is well under way a haziness sets in along its edge which befuddles its outline and effectually hides what is going on within it. When at last the screen clears away the cap is found to be reduced to its least dimensions. Such obstructing sheet looks to be more of the nature of mist caused by the excessive melting of the cap. Unfortunately, there are here no patches of blue-green to test a possible partiality in its behavior over such tracts; nor has similar action ever yet been remarked in the case of the cap of the southern hemisphere. Regular recurrence at the appropriate season of the planet’s year, together with extensive action at the time, takes this springtide mist to some extent out of the domain of weather into that of climate. For it prevails all round the cap and repeats itself in place as each fresh spring comes on. At least it has done so for the past three oppositions at which it has been possible to observe well the arctic zones. It is thus both general in its application and fixed in its behavior. Nevertheless, it betrays something of the fickleness which characterizes that more inconstant thing: weather. For it comes and goes, one thinks for good, only to find it there again some days later. Not less captious is the meteorologic action shown in the making of the new polar cap. When the northern one starts to form, vast areas of frost are deposited in a single night. These, however, are not permanent. The ground thus covered is during the next few days again partially laid bare. Then a new fall occurs, hiding the surface a little more completely than before, and the lost domain is more than regained. By such wavelike advance and recession the tide of frost creeps over and submerges the arctic regions as the late summer passes into the autumn. In this alternate coming and going with succeeding days, we have an unsteadfastness of action most fittingly paralleled by our own weather. It would seem that local causes there as here are superposed upon the orderly progress of the seasons and though at the on-coming of the autumn the cold is steadily gathering strength, nevertheless warm days occur now and then to stay its hand, only to be succeeded in their turn by frosts more biting than before. Even on Mars nothing in the way of weather is absolutely predicable but impredicability. CHAPTER IX MOUNTAINS AND CLOUD