Pagina:Scientia - Vol. VII.djvu/10

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2 scientia
state. We are left, therefore, with water as the only substance we know which could give rise to this phenomenon.
7. The great melting which the polar snows undergo shows their amount not to be considerable, in spite of their extent, and indicates the deposition to be scant.
8. The surface is divided into reddish ochre and blue green tracts, the reddish green being very much the more extensive. This has the look of our reddish deserts and from its general behaviour points to that as its constitution.
9. The blue green areas, which used to be taken for water expanses, are now known not to be such because they are seamed with lines and spots permanent in place, which could not be the case were they lakes or seas.
10. They undergo a change with the Martian seasons, fading out during the winter months and deepening in tint during the summer ones. They behave, in short, like vegetation, and all the evidence points to this as their character.
11. The fact that the caps melt and reform shows, from what we have said above, the presence of water vapor in the Martian atmosphere.
12. This water vapor has registered itself on spectrograms taken by a member of the staff, Dr. Slipher. Spectra of the moon and Mars at the same altitude were recorded on the same plates and the density of the resulting water vapor band «a» proved more pronounced in the Martian spectra. Eight plates were used in all.
13. These plates have been measured by Professor Frank W. Very, giving as his result that the average layer of precipitable water in the air on Mars may be taken as 14 millimeters, while the mean value to the Earth is probably three or four times as great.
14. The fact of change upon the planet’s surface confirms the presence of a Martian atmosphere.
15. The limb light seen around the edge of the disk affords testimony to the same.
16. The planet’s low albedo proves the density of this atmosphere to be much less than our own.
17. The apparent evidence of a twilight goes to confirm this.
18. The greater melting of the south polar cap as compared with the north one indicates that the deposition