The Moon—Photographed at the Lowell Observatory.
So sealike, indeed, was their look that the first astronomers to note them took them unhesitatingly for water expanses. Thus the moment the telescope brought the Moon near enough for map making of it we find the dark patches at once designated as seas. The Sea of Serenity, the Sea of Showers, the Bay of Rainbows, speak still of what once was supposed to be the nature of the dark, smooth, lunar surfaces they name. Suggestively, indeed, in an opera glass do they seem to lap the land. The Lake of Dreams fore-shadowed what was eventually to be thought of them. With increasing optical approach the substance evaporated, but the form remained. It was speedily evident that there was no water there; yet the semblance of its repository still lurked in those shadows and suggests itself to one scanning their surfaces to-day. If they be not old sea bottoms, they singularly mimic the reality in their smooth, sloping floors and their long, curving lines of beach. Their strange uniformity shows that something protected them from volcanic fury while the rest of the lunar face was being corrugated. This preservative points to some superincumbent pressure which can have been no other than water. Lava-flows on such a scale seem inadmissible. What these surfaces show and what they do not show alike hint them sea bottoms once upon a time. In the strange chalk-like hue of the lunar landscape they look like plaster of Paris death-masks of the former seas.
A like history fell to the lot of the surface features of Mars. There too, as soon as the telescope revealed them and their permanency of place, the dark patches upon the planet’s face were forthrightly taken for seas, and were so called: the Sea of the Sirens and the Great Red Sea. Such they long continued to be deemed. The seas of Mars held water in theory centuries after the idea of the lunar had vanished into air. At last, ruthless science pricked the pretty bubble analogy had pictured. Being so much farther off than the Moon, it was much later that their true character came out. Come out it has, though, within the last few years. Lines—some of the so called canals—have been detected crossing the seas, lines persistent in place. This has effectually disposed of any water in them. But here again something of semblance is left behind. They are still the darkest portions of the planet, and their tint changes in places with the progress of the planet’s year. That their color is that of vegetation, and that its change obeys the seasons, stamp it for vegetation in fact. Thus these regions must be more humid than the rest of Mars. They must, therefore, be lower. That they are thus lower and possess a modicum of water to-day marks them out for the spots where seas would be, were there any seas to be. As we know of a vera causa which has for ages been tending to deplete them, extrapolation from what is now going on returns them the water they have lost and rehabilitates their ancient aquatic character. To the far-sight of inference, seas they again become in the morning of the ages long ago when Mars itself was young.
Nor is this the end of the evidence. When we compare quantitatively the areas occupied by the quondam seas on Mars and on the Moon, we find reason to increase our confidence in our deduction. For the smaller body, the Moon, should have had less water relatively, at the time when the seas there were laid down, than the larger, Mars. Because from the moment its mass began to collect, it was in process of parting with its gases, water-vapor among the rest, and, as we shall see more in detail in the next chapter, it had from the start less hold on them than Mars. Its oceans, therefore, should have been less extensive than the Martian ones. This is what the present lunar Mare seem to attest. They are less extended than the dark areas of Mars. A fact which becomes the more evident when we remember that the Moon has long turned the same face to the Earth. Her shape, therefore, has been that of an egg, with the apex pointing toward our world. Here the water would chiefly collect. The greater part of the seas she ever had should be on our side of her surface, the one she presents in perpetuity to our gaze.
It is to the heavens that we must look for our surest information on such a cosmic point, because of the long perspective other bodies give us of our own career. Less conclusive, because dependent upon less time, is any evidence our globe can offer. Yet even from it we may learn something; if nothing else, that it does not contradict the story of the sky. To it, therefore, we return, quickened in apprehension by the sights we have elsewhere seen.
The first thing our sharpened sense causes us to note is the spread of deserts even within historic times. Just as deserts show by their latitudinal girdling of the Earth their direct dependence upon the great system of planetary winds, as meteorologists recognizingly call them, so a study of the fringes of these belts discloses their encroachment upon formerly less arid lands. The southern borders of the Mediterranean reveal this all the way from Carthage to Palestine. The disappearance of their former peoples, leaving these lands but scantily inhabited now, points to this; because other regions, as India, which still retain a waterful climate, are as populous as ever. Much of this is doubtless due to the overthrow of dynasties and the ensuing lapse of irrigation, but query: Is it all? For we have still more definite information in the drying up of the streams which have left the aqueducts of Carthage without continuation, as much to water on the one hand as to its drinkers on the other. Men may leave because of lack of water, but water does not leave because of dearth of men to drink.
Recent search around the Caspian by Huntington has disclosed the like degeneration due to encroaching desertism there. Indeed, it is no chance coincidence that just where all the great nations thrived in the morning of the historic times should be precisely where populous peoples no longer exist. For neither increasing cold nor increasing heat is responsible for this, seeing that no general change has occurred in either. Nor were they particularly exposed to extermination by northern hordes of barbarians. Egypt as a world power died a natural death, and Babylonia too; but the common people died of thirst, indirect and unconscious and not wholly of their own choosing. Prehistoric records make this conclusion doubly sure, by lengthening the limit of our observation. Both extinct flora and extinct fauna tell the same tale. In the neighborhood of Cairo petrified forests attest that Egypt was not always a wiped slate, while the unearthed animals of the Fayum bear witness to water where no water is to-day.
Petrified bridge, third petrified forest, near
Adamana, Arizona—Photograph by Harvey.