Page:Popular Science Monthly Volume 75.djvu/538

534 the lighted rim toward the center. They are the beginnings of those spoke-like markings the methodical oddity of which makes their actuality so difficult of belief. They seem a thought too peripherally positioned to be other than optically evolved. Their recurrent showing in the same places marks them as facts, however, and as such we must regard them. Now when we consider them in the light of Venus's meteorology their cause at once suggests itself. And with this index-finger to guide us we perceive that far from being surprising they are just the phenomena we ought to expect. For consider the surface indraught along the bounding rim of constant sun-exposure. With the immense temperature gradient which exists between the day and the night side of Venus, the power of these winds must be enormous. Being essentially surface ones, they must sweep the face of the planet with irresistible force and, what is more, having once found a pathway of preference, must from the general unchangeableness of the conditions continue to follow it perpetually. For the only thing to alter their direction, the libration, is from the circularity of Venus's orbit negligibly small. Sweeping in originally through valleys or mountain passes, the points offering the easiest access, they must eventually have polished the surfaces over which they passed to a differentiation of appearance visible even across millions of miles of intervening space. Essentially surface currents at the rim, they would become less and less so as they neared the center of the lighted side and furthermore would converge as they approached it. They would seem to us to narrow and become at the same time less salient as they advanced. This is just what their spoke-like character shows. Thus the peculiar look of the Venusian markings proves to be in exact keeping with what the conditions demand, and by so doing bears testimony that those conditions actually exist.

Not less strange on its face and equally interesting for its disclosure is another phenomenon connected with the planet which also has been deemed incredible—the exceeding brightness of Venus's disk. Her great luster is, as we saw above, in part attributable to her proximity first to the sun and secondly to us. But this is not the sole cause of it. Though a part of her splendor is due to her position, a part is her own. Her intrinsic brightness, her albedo, as it is called, has been found by Müller, of Potsdam, who has made the last and most authoritative determination of it, to reach the excessive figure of.92 of absolute reflection. This figure has seemed to many impossible, but we shall see from consideration that it simply reflects the conditions.

The rising currents on the sunward side must from their great heat be capable of holding much water-vapor in suspension. This they would take over with them in large part to the night side and becoming chilled there deposit it as snow. Being cold on their return they would reenter the warm side relatively dry and thus be fitted to act again as water-carriers from that side to the other. This process of depletion