CHANGES IN THE ASPECT OF MARS. 533 



construction of bathymetric charts of our oceans ; and thirteen years 

 ago I described the form of the Atlantic Ocean at four thousand me- 

 tres 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 atmos- 

 pheric 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. Notwith- 

 standing the tenuity of these channels, they are not less than one hun- 

 dred and twenty kilometres in breadth, while some of them are fully 

 forty-eight hundred kilometres long.* These results were at first re- 

 ceived 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 



