Whether or not astronomers agree with Prof. Lowell's Martian theories, it cannot be denied that he has been by far the most indefatigable observer of our planetary neighbor. His studies have been elaborate and painstaking, and have involved not only the expenditure of years of time, but the erection of a private observatory in an atmosphere peculiarly fitted for his work. Based upon this foundation, any book of his on the subject deserves somewhat more consideration than the passing review which usually falls to the lot of a popular exposition of an important scientific investigation.
In the first place, Prof. Lowell is a staunch believer in the habitability of Mars. His conclusions, reached after a minute study of the puzzling surface markings of the planet, are based on a wealth of ingenious reasoning that cannot but appeal to the romantically inclined. In the following brief paragraphs we have endeavored to present in succinct form the theory which Prof. Lowell has formulated, and the plausible arguments he has advanced to uphold that theory.
Viewed through a telescope, Mars appears as a disk crowned with white spots and covered with blue-green and reddish ocher patches. Upon the fluctuations of these markings Prof. Lowell bases his conclusions of the habitability of the planet. Most prominent of all the markings are the white spots that cap the poles. They are the most important evidence of the planet's constantly changing condition, for they come and go just as our own polar snows wax and wane. In the depth of winter they stretch over much more than the polar regions, extending down to 60 degrees and even to 50 degrees of latitude north or south as the case may be, then dwindling until, by midsummer, they extend only 5 or 6 degrees across. A three-inch glass is sufficient to disclose these modifications. It was early surmised that Martian caps must be composed of ice and snow, a theory which Prof. Lowell substantiates by pointing out that as the Martian cap melts it is surrounded by a deep blue band, which keeps pace with the shrinking cap and is clearly the product of its disintegration. This ribbon of blue conclusively shows that not gas but water is the substance of which the caps are composed.
If the caps melt, they must clearly pass into a gas, which means that Mars must have an atmosphere. That atmosphere, it is safe to conclude, is composed primarily of water vapor. Corroborative evidence of the presence of Martian air is shown by the existence of clouds, rare though they may be. Other evidence is afforded in the limb light, a phenomenon which may be described as a brilliant obliteration near the edge of the disk, an obliteration which suggests a veil drawn between us and the planet, and which can be caused only by air or haze. Evidence has also been gathered of the existence of a twilight, which would indicate the presence of a thin high air more rarefied than prevails on our highest mountain peaks. That the atmosphere must be thin is proven by the uninterrupted view of the Martian disk in all zones.
Of the blue-green and reddish ocher patches to which reference has been made, it may be stated in a general way that of the two the reddish-ocher tint predominates, occupying as it does, five-eighths of the disk. Early in the history of Martian observation the blue patches were taken for seas, and received names in keeping with the conception. Thus, we have the Sea of Serenity, the Sea of Vapors, and the like. The initial doubt of their watery nature was cast by their change in aspect, a change first noticed by Schiaparelli. The coup de grace to the old belief was given when Pickering and Douglass found that the dark areas were traversed by permanent lines. If the blue-green areas are not seas, what are they? According to Prof. Lowell, only vegetation can account for their singular fluctuations. He finds that in their color (blue-green) the dark areas exactly typify the distant look of our own forests. If the changes are vegetal, they must occur at the proper season of the planet's year. Generally speaking, it may be said that certain regions pass from ocher to blue-green in a few weeks at a season corresponding with the Martian spring. Conversely, the blue-green regions are converted into ocher with the coming of autumn. Mars owes its fiery tint to the great ocher stretches. Land the ocher regions have generally been taken for, and land they undoubtedly are. Indeed, they seem to be nothing but deserts. Their pure salmon hue is characteristic of the Sahara desert and of the desert of northern Arizona.