170 NOTES ON MAES. 



where it was distributed to all astronomers and to many others 

 interested. 



Astronomers almost never see clouds on the sunlit portions of the 

 planet. If they exist as thin cirrus clouds, we should perhaps be 

 ujiable to see them. Heavy clouds, if they covered a large enoug-h 

 area of the planet, would be visible. The}^ would have to cover per- 

 haps a million square miles before they could be recognized with cer- 

 tainty. But on this earth they cover far more area than that, and 

 therefore, as we do not see them on Mars at all, we conclude that the 

 planet is extremely dry. As a matter of fact, it has no oceans and no 

 surfaces that are positively identified as permanent water surfaces. 

 Some observers even doubt that the planet is warm enough to permit 

 the existence of water. The only two indications of water, however, 

 are in the polar caps of snow and the clouds, such as this one, which 

 becomes visible to us as the sun sets on the region above which they 

 float. These clouds stand above such regions like the peaks of high 

 mountains, and receive the last rays of the setting sun, when all 

 beneath is dark. They therefore appear to us as bright spots against 

 a dark background or as bright points extending from the sunlit por- 

 tion of the planet out into the region of night. 



The first observation of a phenomenon of this kind was made at the 

 Lick Observatory in 1890. A few were seen in 1892 at a number of 

 observatories. Over 350 were observed and studied at this observa- 

 tory in 1894, and many were seen in 1896. The result of the study of 

 several hundred of them is given in Vol. I of the Annals of the Lowell 

 Observatory. By that research it became evident that man}^ of them 

 were formed at sunset. In character they were probably like our 

 dense cumulus clouds. Their average elevation was much greater 

 than our cumulus clouds, being several miles on an average, and one 

 was seen at least as high as 15 miles above the surface, that of Novem- 

 ber 25, 1894. In that case the cloud was seen near the sunrise termi- 

 nator, as it is called — that is, the north and south line on the planet at 

 which the sun was rising. If we were searching for signals, the cloud 

 of November 25, 1894, would be a very much more striking case than 

 anything seen before or since. On the night of November 25 this 

 cloud was seen as a white spot against the dark background of the 

 unillumined surface beneath. It was 100 miles across and at least 15 

 miles above the surface. It remained in that point over half an hour, 

 then suddenly disappeared. On the following night it was only 8 miles 

 high. Instead of remaining constant!}^ as a bright spot, it appeared 

 and disappeared. The first appearance lasted sixteen minutes; after 

 four minutes it disappeared; it came again for only a moment, and after 

 six minutes more it again appeared for two and one-half minutes. Then 

 followed an absence of three minutes, presence for two minutes, 



