culiar climate where it thrives best, and beyond certain limits it ceases to exist. The successive zones of vegetation, as we recede from the equatorial regions, have sometimes been supposed to be represented by the different altitudes upon the mountains under the equator, as it is evident we have in ascending from the valleys to their snow-capped summits, every variety of temperature. The analogy fails however in one essential point, for as we ascend the mountains the pressure of the atmosphere is continually diminished and it is evident that less nutriment is thus afforded for the growth of the plant. The influence which the variations of climate alluded to, must have upon vegetation is very evident, thus in many parts of Siberia, wheat and rye are raised upon a soil which is constantly frozen at a depth of three feet, while in Iceland, where the mean temperature of the year is much warmer, and the winter's cold but inconsiderable, it is not possible to raise any of the ceralia or common grains, as the low summer temperature does not suffer them to ripen. It is for the same reason that the vine does not flourish in England, for although it can endure a tolerably great degree of cold, yet it requires a hot summer to make the fruit ripen, and yield a drinkable wine. There is no subject connected with meteorology which requires a more careful, and studied investigation than that of climates. So many causes influence the temperature of the air, and some of them are so variable, that no labor short of a well conducted series of observations, extending through a long course of years can give a satisfactory result. In the brief account we have given, we have been able to present little else than the leading facts, and must refer the reader to the writings of Leslie, De Candolle, Mirbel, and Humbolt, for further information. TABLE OF TEMPERATURES. 157 TABLE Exhibiting the mean temperature of various places compiled principally from the observations of Baron Alex. Von Humboldt. 56 CHAPTER V. Optical Phenomena. Why do those cliffs of shadowy tint, appear Campbell. In the present chapter we shall describe and explain the general optical appearance of the sky, and some of the more striking optical phenomena connected with our present subject. When the rays of the sun strike the minute particles of air, which, according to circumstances, may be more or less dense, or charged with watery vapor, they are either reflected, or transmitted; in either case sometimes returning the most beautiful colors. It is a fact to well known to need much illustration from us, that light, whenever it is refracted by any medium, such as glass or water, is always separated into the prismatic colors, whenever the surfaces of the medium are curved, or inclined to each other. It is not however, so generally understood, that these different colored rays have different powers of penetrating through various media, and that they move with different velocities. This however, is susceptible of demonstration, and it is to this that the beautiful colors of an autumnal sunset are owing. The red, violet and orange rays have the greatest velocity, and penetrate the thick dense strata of horizontal air, with the greatest facility, giving us the rich and brilliant hues of sunset and sunrise, tinging the morning and evening clouds with glowing red, and gold; and the sober twilight, with that purple fading into gray which is assumed when the ruddy glare of sunset is tempered by the azure of the sky. Since the red and yellow rays which compose white light, are transmitted by the air, unattended by the blue rays, it follows that these latter must be reflected, hence the beautiful blue of the sky, and the bright azure which tinges the distant mountains when viewed through a considerable body of intervening air, and especially, when charged with watery vapor Perhaps this one feature, which so mellows down the distant outlines of the hills and buildings, is the most pleasing feature of the landscape. It is from strict attention to the phenomena dependent upon this principle, that the artist derives his pleasing skill in picturing objects of varying distance, introducing skillfully the color of the intervening air. How simple, and yet how beautiful are the various contrivances which administer, not to the wants merely, but to the pleasures of man. It is the same simple cause which tints the bright blue sky, and its beautiful clouds, here piled in snowy masses, and there sundered into a thousand fleecy shapes; which lights the west with a golden glow, and fringes the extended clouds that skirt the horizon with the brightest hues of red and gold; and it is owing to the peculiar nature of the red rays of the spectrum, that the sun appears a dull red globe when viewed through air highly saturated with watery vapor, or through clouds and fogs. When the rays of the sun strike upon a cloud, they are copi ously reflected, but partly absorbed by the minute suspended globules, and the quantity of light which penetrates through the nebulous medium is always much less than what traverses an equal body of air, and this gives the clouds their varying shades of color. That the color of the sky is owing to reflected light, is sufficiently evident from the fact, that it becomes darker and darker, as we ascend into the higher regions of the atmosphere, through which, the blue rays find a ready passage. Were it not for the reflecting power of the atmosphere, and the clouds, we would have no softening of the day into night, as now, by the twilight; but instantly, at sunset, darkness would veil the earth, and every cloud that obscured the sun would cause a total eclipse. The tint of the sky is deeper in the torrid zone than in high latitudes, and in the same parallel it is fainter at sea than on land, this may be attributed to the aqueous vapor continually rising towards the higher regions of the air from the surface of the sea. The presence of much moisture is also easily detected by the |