COBRESP ONDENGE. 



Ill 



CORRESPONDENCE. 



A COREECTION. 

 To the Editor of The Popular Science MontUy. 



SIR: Please allow me to correct some 

 errors in the notice (on page 760 of 

 this journal for October) of the paper on 

 "American Ganoids," read at the Detroit 

 meeting of the American Association for the 

 Advancement of Science. 



The very young gar-pike (Leptdosteits), 

 less than an inch long, has only one tail ; a 

 symmetrical organ like that of existing 

 Amphioxus and Polypterus, and the fossil 

 Glyptolcemus. 



While from one to ten inches long, the 

 growing gar manifests a lower lobe of the 

 caudal. In this state it resembles the ex- 

 isting sturgeons and sharks, and many fos- 

 sil Ganoids. 



The filamentary original tail gradually 

 decreases and filially disappears, while the 

 lower lobe increases and becomes the 

 functional tail of the adult Lepidosteus 

 and Amia. In this respect, therefore, 

 these forms are modern types of an an- 

 cient group. 



In describing the peculiar vibratory 

 movement of the caudal filament of the 

 young gar, I compared it to the rapid vi- 

 bration of the tail in many if not all ser- 

 pents, and notably in the rattlesnake, and 

 suggested that, in view of the ball-and- 

 socket articulations of the vertebrae of Lep- 

 idosteus and some other reptilian features, 

 the resemblance between the motions of 

 Lepidosteus and Crotalus may have a deeper 

 origin and significance than mere functional 

 similarity ; that they may have had a com- 

 mon ancestry not very remote. But I had 

 no idea that " the ancestor of the gar was 

 a reptile." 



This correction seems to me the more 

 desirable, since the other paper noticed by 

 you (on the Sirenia) was chiefly to show 

 that a retrograde metamorphosis had taken 

 place with that group. 



Burt G. Wilder. 

 Ithaca, N. T., September 27, 1875. 



yOEESTS AND RAINFALL. 



To the Editor of 'the Popular Science Monthly. 



While recently traveling among the 

 mountains of this State, the threatening 

 approach of a storm obliged me to find a 

 shelter, whence my attention busied itself 

 in watching the clouds gathering upon the 

 slopes that reached at least two thousand 

 feet above the valleys. 



Some portions of them, I observed, be- 

 came quickly covered ; others more slowly. 

 In due time the storm broke away, and, re- 

 lieved partially of their watery burdens, the 

 clouds commenced to lift and move off, but 

 some more tardily than others. Moreover, 

 I remarked that, where they had first col- 

 lected, there they remained the longest, and 

 that those parts of the acclivities concealed 

 the last were the first to become visible. 



Such a singular coincidence led me on 

 further to the consideration of its cause. I 

 think it may be extracted from the follow- 

 ing facts : 1. The day had been very warm, 

 as had also been the weather for a week 

 before. 2. Of those portions of the slopes 

 that had become hidden, the timbered lands 

 were the first and, as mentioned above, the 

 last to be seen again ; the contrary happen- 

 ing to the rock-exposures. 3. The valley 

 in which I was is formed by mountains over 

 four thousand feet above sea-level, their 

 opposing acclivities being very near to each 

 other. It is therefore narrow, and it was 

 shielded from the cooling influences of winds 

 outside. 4. The radiation of heat from the 

 bare sides and precipices. 



Generalizing the conclusions that may be 

 drawn from these, it may be said that some- 

 times clouds passing over barren surfaces, 

 like some of those I had been viewing, 

 will become lightened as the cohesion of 

 their particles is weakened by the warmer 

 ascending currents of air ; they may be 

 dispersed, and, even if they settle down, 

 will be more likely to rise again before 

 those covering forests. 



With the latter it will be otherwise. 

 Every leaf, like a miniature sun-shade, pro- 



