Page:Popular Science Monthly Volume 75.djvu/537

Rh the line as it were. Tidal action depends, for the time necessary to produce a given result, on the square of the radius of the body acted on and as the sixth power of its distance from the exciting cause. In consequence for solar action the nearer planets would show its effect first. Now Mercury already turns the same face always to the sun; the earth, as we know, does not. Venus comes between the two in distance and might therefore a priori do either, depending upon how long the action has been going on. That she agrees with Mercury in continuously staring at the sun thus affords valuable evidence on the general evolution of the solar system.

Interesting as this information is, it is second to what we learn in consequence about the body itself. To have the same hemisphere exposed everlastingly to sunlight while the other is in perpetuity turned away, must cause a state of things of which we can form but faint conception from what we know on earth. Baked for æons without let-up and still baking, the sunward face must, if unshielded, be a Tophet surpassing our powers adequately to portray. And unshielded it must be, as we shall presently see. Reversely, the other must be a hyperborean expanse to which our polar regions are temperate abodes. For upon one whole hemisphere of Venus the sun never shines, never so much as peeps above the star-studded horizon. Night eternal reigns over half of her globe! The thought would appall the most intrepid of our arctic explorers, and prevent at least everybody from going to the pole; or rather what here replaces it "through the dark continent."

Deduction from our known premises enables us to go further in sketching the picture of Venus's globe. Venus we know has an atmosphere. The effects of it are patent at the times when she passes between, or nearly between, us and the sun. She is then seen haloed by a rim of light due, as Wilson has shown, to reflection chiefly, not to refraction as was formerly supposed, from an atmosphere about her. Now the intense heating to which the center of her sunward side is exposed must necessarily expand the air there, causing it to rise funnel-wise up in a world-wide western cyclone. To fill the space thus depleted currents must set in toward the center from all points of the compass, continuing out to the lighted rim. Their place in turn would be occupied by surface indraughts from the dark side. Meanwhile the heated air would spread like an umbrella round into the cold hemisphere there to descend and replace the outgoing superficial current back to the sunlit face. A regular aerial round of travel is thus started, which is the same forces that began it must keep up. The course is surface-wise from the dark to the illuminated hemisphere; aloft from the sunlit to the night one.

Now this simple, regular and reliable meteorological service explains a feature of the visual observations which has deterred many timid souls from crediting their reality. One of the most striking features of Venus's disk are the tongues of shading that make in from all parts of