THE PLANET VENUS 533 



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 dis- 

 tance and might therefore a priori do either, depending upon how long 

 the action has been going on. That she agrees with Mercury in con- 

 tinuously staring at the sun thus affords valuable evidence on the gen- 

 eral 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 ex- 

 posed everlastingly to sunlight while the other is in perpetuity turned 

 away, must cause a state of things of which we can form but faint con- 

 ception from what we know on earth. Baked for aeons 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. Eeversely, the other must be a hyper- 

 borean 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 aj)pall 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 atmos- 

 phere. The effects of it are patent at the times when she passes be- 

 tween, 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 refrac- 

 tion as Avas 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 cur- 

 rents must set in toward the center from all points of the compass, con- 

 tinuing 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 surf ace- 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 



