more than, and probably less than, one seventh of an atmosphere. In consequence, on a rise of temperature the frozen carbonic acid gas would there pass practically straight from the solid into the gaseous state. Now, from the existence of the surrounding polar sea, we remark that in the substance composing the polar caps of Mars this does not occur. A considerable portion of it is always in the transition state of a liquid. Carbonic dioxide would not thus tarry : water would.

Third: from the curve of metamorphosis, it is evident that the temperature necessary to freeze the gas under the pressure of one seventh of an atmosphere must lie between -100 degrees C. and -200 degrees C., if not lower. -200 degrees C. is, so far as we can judge, about the temperature of inter-planetary space, or what would be the temperature of the night side of Mars were the planet destitute of atmosphere. But there is an atmosphere on Mars, and, even if there were not, on melting the carbonic dioxide would itself make an atmosphere. This would instantly raise the temperature, and under any rise in temperature the congealing of the gas at once becomes an impossibility. The gas itself thus suggests its own refutation.

There is no such apparent objection to water. With an atmosphere properly constituted (and there is nothing to show that the Martian atmosphere is not so constituted), the temperature might easily rise high enough to melt ice. We may therefore conclude water to be the most probable solution of the question.

With such more or less solid ground to stand on, we may now go on to describe the behavior of the cap as constituted of snow. Whether we call it snow-cap or ice-cap is immaterial, as, although it would probably be deposited as hoar-frost rather than as snow in the first instance, owing to the thinness of the Martian air, the latter end of either form of the substance would be much the same,--glacier-ice.

It will, be interesting to examine more in detail the annual history of the ice-cap, especially as this history was unrolled before us last year more minutely than has been the case for the last fifteen years, and than will be the case for fifteen years to come. This was due not only to the relative proximity of the planet during the last opposition, but to the further fact that its south pole was tilted toward us at maximum angle. The vicissitudes which the polar cap underwent stood, in consequence, remarkably well displayed. To such advantage were they seen that it has been possible to construct a map of the Martian south circumpolar regions to a degree of detail such as has never been possible before, and which I have accordingly done. It will be seen from it (on the opposite page) how much farther advanced is our knowledge of the Martian south pole, and the regions about it, than is our knowledge of either of our own.

It is also pleasing to remember that during this our polar expedition we were not frost-bitten for life, nor did we have to be rescued by a search party. We lived not unlike civilized beings during it all, and we actually brought back some of the information we went out to acquire.

On examining the chart in which the successive appearances of the southern ice-cap are depicted at different times, from June 3 to October 13, or, in terms of the Martian time of year, from May 1 to July 15, the first point to strike one is that the cap was during its whole existence eccentrically placed with regard to the geographical pole of the planet. In other words, the pole of rotation and the pole of cold did not coincide. The latter lay on the average some six degrees distant from the former. This shows that the isotherms in the southern hemisphere of Mars do not coincide with the parallels of latitude.

The manner of the cap's melting further shows that differences of level exist in it. For, in addition to melting round its edge, the cap proceeded to melt asymmetrically. On the first night that Professor W. H. Pickering observed it, on May 22, with the six-inch telescope, he suspected a rift crossing the cap from longitude 330 degrees to longitude 170 degrees. This rift grew more and more evident, until, in the early part of June, it was unmistakable. It grew in visibility chiefly from actual growth in size. On June 6 it was estimated, on a scale of ruled lines made for the purpose, to be about 100 miles wide. On June 15 it was similarly found to measure 220 miles.

Meanwhile an interesting phenomenon occurred in the cap on June 7. On that morning, at about a quarter of six (or, more precisely, on June 8, 1h. 17m., G. M. T.), as I was watching the planet, I saw suddenly two points like stars flash out in the midst of the polar cap. Dazzlingly bright upon the duller white background