Popular Science Monthly/Volume 8/November 1875/Correspondence
To the Editor of The Popular Science Monthly.
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 (Lepidosteus), less than an inch long, has only one tail; a symmetrical organ like that of existing Amphioxus and Polypterus, and the fossil Glyptolœmus.
While from one to ten inches long, the growing gar manifests a lower lobe of the caudal. In this state it resembles the existing sturgeons and sharks, and many fossil 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 ancient group.
In describing the peculiar vibratory movement of the caudal filament of the young gar, I compared it to the rapid vibration of the tail in many if not all serpents, and notably in the rattlesnake, and suggested that, in view of the ball-and-socket articulations of the vertebrae of Lepidosteus 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 common 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.
FORESTS 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, became quickly covered; others more slowly. In due time the storm broke away, and, relieved 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 collected, 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 following 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 happening 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 sometimes 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, protects a part, small though it be, of the soil from the direct warmth of the sun. Forests thus are like great canopies sheltering from the sun's rays those sections upon which they grow. Lands so covered possess a capacity for holding much moisture. Contained in the leaves and trunks of trees, and more particularly in the spongy moss and numerous streams, it is saved from rapid evaporation, and consequently lowers the temperature of the atmosphere over it.
Vapors, then, attracted toward mountains by gravity, or carried thither by winds, will at times collect first over those sections which are wooded, and will have a tendency to remain there, be condensed, and deposit rain.
It may not be out of place to notice here another fact coming under my observation. Winds sweeping across a country, when they encounter mountains, are crowded against them, and, by the pressure from behind, are forced up along their sides and over their crests. Clouds that are in their paths, and which are borne onward to the slopes of such mountains, are sometimes carried up to and over their tops. Slopes which are destitute of timber present very few obstacles to such a result. Forests, on the other hand, break or lessen the mechanical strength of wind, and so increase the probability of their augmenting the volume of rainfall.