the Surface- Temperature of the Planets. 173 



shall discover a significance in their several albedoes. Three 

 classes stand forth distinct : — (1) those possessing no air ; 

 (2) those with air, but wholly or in part cloudless ; (3) those 

 with a cloud-covering. Into these classes the planets fall in 

 the manner below, while the albedoes the}' respectively present 

 are placed alongside of them. 



I. Airless bodies. 



Albedo. 



Mercury 0*17 



Moon 0*17 



II. Air-enveloped bodies. 



Venus, cloudless, \ medium 0-92 

 Earth, 50 % clouded j air 075 



Mars, cloudless, thin air 0*27 



III. Cloud-canopied bodies. 

 Albedo. 



Jupiter 0*75 



Saturn 0-88 or 0*78 by Struve's 



Uranus 0*73 [latest measures. 



Neptune 0-63 



The albedo of cloud is 0*72. Whence it is clear that cloud 

 -cannot account for the albedo of Venus ; but that it accords 

 with the albedo of the four major planets. That an air- 

 envelope increases the albedo of a planet is witnessed, first, by 

 the greater brilliancy per unit of disk of the Earth, Venus, 

 and Mars as compared with the airless bodies, Mercury and 

 the Moon; and, secondly, by the relative specific brightness of 

 Venus and Mars, together with what has above been found 

 as to that of the Earth. It appears that the denser the air 

 surrounding the planet, the more dazzling the aspect the 

 planet presents. This is undoubtedly due not to the gases 

 themselves, but to the solid or liquid particles the gases 

 support in the shape of dust, ice- particles, or drops of water. 



This testimony of the albedo that Venus is not cloud- 

 covered but atmosphere-hid is corroborative of the observations 

 made by the writer at Flagstaff in 1896 and at Mexico in 

 1897, from which it appeared that the planet's markings were 

 not obscured by cloud but seen as it were through a veil, and 

 which also showed the correctness of Schiaparellr's deduction 

 that Venus, in all probability, turned in perpetuity the same 

 face to the Sun. That she did so was evident from the long- 

 continued observations at Flagstaff and Mexico. Now such 

 a facing always of one hemisphere sunward would cause 

 convection-currents upward in the centre of the disk, and an 



