construction of bathymetric charts of our oceans; and thirteen years ago I described the form of the Atlantic Ocean at four thousand metres below the present surface as "bottle-necked." If, then, we suppose the water of the Atlantic to become absorbed in the profound masses at this moment in process of solidification, in such a way that the level of that ocean shall be depressed by four thousand metres, we shall have at the same time a much smaller surface covered by water, and a narrow and elongated form of the seas, or exactly the conditions which Mars presents. At the same time that the water is thus drunken up, the air also will be undergoing absorption. All the rocks are aerated. We know what trouble we have in driving the air from even the most compact rock of which we wish to obtain the density with precision. Since the different mineral masses become aerated while they are becoming moist, and consequently, while they are cooling, the atmospheric strata should undergo a progressive decrease. It is, therefore, natural that the atmosphere of Mars should be much thinner than that of the earth; and that is an excellent condition for the telescopic study of the planet.
For the earth, geology furnishes a kind of indirect confirmation of this progressing absorption of the atmosphere. The results of the experiments of physicists, of Mr. Tyndall in particular, go to show that a slight augmentation in the thickness of our atmosphere or in the proportion of vapor it contains, would suffice to cause the solar heat to be stored in larger quantity and wasted more slowly; that is, in short, would make what we call climates disappear—a warm and nearly equable temperature prevailing over all the earth. Now, one of the most remarkable characteristics of the ancient geological periods is just this absence of climate, which is indicated by the uniformity of fauna and flora over the whole planet; and this confirms our opinion that the atmosphere once formed a much thicker bed than it does now.
While there thus exist traits in common between Mars and the earth, a strange motive of interest lies in the existence on the surface of the former globe of very important details of structure which are without analogy with us. M. Schiaparelli first perceived, in 1877, in the continents of Mars, which had been till then very large and without solution of continuity, a system of dark channels, often very slender, which divided the surface into a multitude of lands isolated and separated from one another like the meshes of a net. Notwithstanding the tenuity of these channels, they are not less than one hundred and twenty kilometres in breadth, while some of them are fully forty-eight hundred kilometres long. These results were at first received with incredulity by astronomers, who were afterward, however, constrained to recognize their rigorous exactness. The works of the distinguished director of the observatory at Milan upon this subject, of which the last one, relative to the opposition of 1879–'80, constitutes a
