We can all remember Croll’s “Climate and Time,” a book which can hardly be overpraised for its title and which had things worth reading inside, too. It had in consequence no inconsiderable vogue at one time. It undertook to account for glacial epochs on astronomic principles. It called in such grand cosmic conditions and dealt in such imposing periods of time that it fired fancy and almost compelled capitulation by the mere marshalling of its figurative array. Secular change in the eccentricity of the Earth’s orbit, combined with progression in the orbital place of the winter’s solstice, was supposed to have induced physical changes of climate which accentuated the snowfall in the northern hemisphere and so caused extensive and permanent glaciation there. In other words, long, cold winters followed by short, hot summers in one hemisphere were credited with accumulating a perpetual snow sheet, such as short, warm winters and long, cold summers could not effect. [Illustration: MARS. NORTH POLAR CAP. SOUTH POLAR CAP. At maximum full extent of white At maximum white At minimum inner circle At minimum nothing] Now it so happens that these astronomic conditions affecting the Earth several thousand years ago, are in process of action on one of our nearest planetary neighbors at the present time. The orbit of Mars is such that its present eccentricity is greater than what the Earth ever can have had, and the winter solstice of the planet’s southern hemisphere falls within 23° of its aphelion point. We have then the conditions for glaciation if these are the astronomic ones supposed, and we should expect a southern polar cap, larger at its maximum and still more so, relatively, at its minimum, than in the opposite hemisphere. Let us now look at the facts, for we have now a knowledge of the Martian polar caps exceeding in some respects what we know of our own. The accompanying diagrams exhibit the state of things at a glance, the maximum and minimum of each cap being represented in a single picture and the two being placed side by side. It will be observed that the southern cap outdoes its antipodal counterpart at its maximum, showing that the longer, colder winter has its effect in snow or hoar-frost deposition. But, on the other hand, instead of excelling it at its minimum, which it should do to produce permanent glaciation, it so far falls short of its fellow that during the last opposition at which it could be well observed, it disappeared entirely. The short, hot summer, then, far exceeded in melting capacity that of the longer but colder one.