THE PLANETS MARS AND VENUS 357 



snow area there on either February 12 or 13. The writer does 

 not believe that the hoarfrost is ever seen far from the sunrise 

 terminator, and he would suggest that what Professor Lowell 

 interprets as hoarfrost is described here as cloud. The appear- 

 ance noted here has a white pearly lustre, clearly fainter than 

 the snow at the north pole, but brighter and much whiter than 

 the desert areas of the planet. 



White cloud is occasionally seen in the equatorial regions 

 close to the limb, but when it gets nearer the center of the disk 

 it turns yellow, owing doubtless, as previously stated, to its thin- 

 ness, and to the fact that we see the soil between separated small 

 clouds. Our own cumulus clouds often have a bluish tint, even 

 near thin centers, when they are forming or dissolving, and are 

 therefore thin enough for us to see our blue sky through them. 



It is the writer's impression that the meteorology of Mars 

 resembles very closely that of the earth, much more closely than 

 has heretofore been supposed. Clouds are numerous on both 

 planets but they are more so in the daytime with us, and possess 

 greater density. It is believed for theoretical reasons that the 

 northerly and southerly components of their winds are more 

 marked than ours, that their daily range of temperature is greater, 

 and that their precipitation usually occurs at night, but except in 

 these respects the meteorology of the two planets is practically 

 identical. * * * 



For many years Martian observers have been in the habit of 

 reporting marked changes as occasionally occurring upon the 

 planet, but the writer believes that a considerable proportion of 

 these, perhaps a third, were due simply to shifting clouds. Thus 

 the numerous and marked changes in size of the southern polar 

 cap at this opposition are almost certainly due to nothing more 

 complex than the formation and dissolution of cloud areas. The 

 sudden appearance of two lakes and four canals in the Protei 

 Regio on December 31, where on the previous night only a whitish 

 area was visible, was presumably due to the clearing away of 

 clouds. Possibly the fact that certain narrow canals were seen 

 south of the Sabaeus in January, which were not seen on Febru- 

 ary 14 to 16, though near the center of the disk, with seeing 12 

 and 10, is due to their having been hidden by invisible, because 

 partially transparent, cloud masses. (April, 1914) 



When any of the large polar marshes, but especially 

 Acidalium, come around the sunrise limb, they are usually fol- 

 lowed at a distance, sometimes as great as 200 miles, by a yellow 

 or white area of considerable size. The former color indicates 

 cloud, and is the commoner of the two. The white probably in- 

 dicates a thin layer of new fallen snow. It must be thin, since 

 it never persists to the central meridian. On October 9, Martian 

 date February 48, a pure white area one-third the size of the 



