luvion brought down by the mighty rivers into this tide-water region is of a humid nature, abounding in organic remains, effluvia or miasmata, noxious to man, are here copiously generated. The western limits of this tract are very distinctly marked by a ledge of primary rocks, over which the rivers fall, and to which, in the northern section, the tide penetrates. Along this line are found Trenton, Philadelphia, Baltimore, Georgetown, Fredericksburg, Richmond, Smithville, Camden, Augusta, Milledgeville, and Columbus. The fact that nearly all the principal cities of the Atlantic States have arisen upon this boundary, from the obvious motive of seeking the head of navigation, affords a striking example of the influence of geological causes in distributing population, and thus determining political rela

tions.

The territory of the United States is traversed by two great systems of mountains, by which the country is distinctly marked into three natural divisions, viz., the Pacific region, the Mississippi Valley, and the Plain just described. The Alleghany or Apalachian system, bounding the Atlantic Plain, has a mean altitude above the sea of about two thousand five hundred feet; of which not more than one-half consists of the height of the mountain ridges above their bases, the adjacent country having an equal elevation above the ocean. Several of the most elevated summits, however, as Black Mountain and Mount Washington, attain an altitude of nearly six thousand five hundred feet. The more lofty and extensive mountain-chain in the western part of the continent, which is known under the various names of Rocky, Oregon, and Chippewyan, is a prolongation of the Andes or Mexican Cordilleras, extending to the Arctic Sea. Rising from a common base of about three thousand feet above the level of the sea, the average height of the summits above the base is estimated at five thousand feet, some of the crests being not less than eight or ten thousand feet above the adjacent country. This mountainchain is five or six hundred miles

from the Pacific. Farther west is the range of the Pacific coast mountains, which stretch northward from California into the Peninsula of Russian America. They are from seventy to eighty miles distant from the coast, and have peaks rising even above the most elevated of the Rocky Mountains, some being estimated at from ten to eighteen thousand feet above the level of the sea. These summits, like those of the Rocky Mountains, are covered with snow, and ascend far into the region of perpetual congelation.

It is geographical features such as these that produce modifications of climate, and as climatic laws are also influenced by the character of the surface, the following general description by our author will not be out of place:

"From the shores of the Atlantic to the Mississippi, there is presented an immense natural forest, interspersed with open and naked plains, called prairies, which are numerous west of the Allegha nies, but very rare on the Atlantic side. The country west of the Mississippi is arid and desert plains, occupying a comparatively lightly wooded; and in the breadth of three or four hundred miles, only a few trees are seen along the margins of the rivers. In that portion of the United States which is inhabited, the land cleared and cultivated does not probably exceed one-tenth part of its surface."

But one of the most striking characteristics of the physical geography of the United States, and which, it will be seen, induces the most remarkable modifications of climate, is the exist ence of those great inland basins of water which lie on our northern frontier. Of so vast an extent are these ocean-lakes, which we shall bring more particularly under notice hereaf ter, that one of them (Lake Superior) has a circuit, following the sinuosities of the coast, of one thousand seven hundred and fifty miles. The physical features of America generally have been cast in large forms. Her rivers are amongst those grand natural features in which she claims the most decided pre-eminence over the other

of terrestrial temperature depended solely on the former class of causes, climates might be classified with mathematical precision; but the effects produced by solar heat are so much modified by local causes, that the climatic features of any region can be determined only by observation. It is a general law, for instance, that the mean temperature of the earth's surface gradually increases from the poles to the equator, but this law is greatly modified by the agency of physical geography. Amongst these local causes, the following are regarded as the principal:-1. The action of the sun upon the surface of the earth; 2. The vicinity of great seas and their relative position; 3. The elevation of the place above the level of the sea; 4. The prevalent winds; 5. The form of lands, their mass, their prolongation toward the poles, their temperature and reflection in summer, and the quantity of snow which covers them in winter; 6. The position of mountains relatively to the cardinal points, whether favoring the play of descending currents or affording shelter against particular winds; 7. The color, chemical nature, and radiating power of soil, and the evaporation from its surface; 8. The degree of cultivation and the density of population; 9. Fields of ice, which form, as it were, circumpolar continents, or drift into low latitudes.

It is these causes that determine the deviations of the isothermal, isocheimal, and isotheral lines from the same parallels of latitude. The isothermal curves represent lines drawn upon a map through all the places on the globe having the same mean annual temperature; and these lines are by no means, as might be expected, regular. Were two travellers, for example, to set out, the one from London and the other from Paris, visiting all the places having the same mean annual temperatures, it would be found that the lines of their routes would not only deviate from the parallels of latitude, but would not be parallel to each other. Thus, the isothermal line or mean annual temperature of Edinburgh, Scotland, strikes the Atlantic coast of North America twelve degrees farther south. Hence the former division of the surface of the earth into five zones, as regards its temperature, has been superseded in scientific inquiries, by a more

having the same mean annual temperature are connected by isothermal lines, the spaces between them are called isothermal zones. Whilst in this arrangement are classed together the mean temperature of the whole year, the application of the same principle to any portion of the year, as the mean winter and summer temperatures, is equally obvious. Thus lines drawn through places having the same summer temperatures are denominated isotheral, and those through points having the same winter temperatures, isocheimal curves. The importance of this classification, more especially as places having the same mean annual temperature often exhibit great diversities in this respect, will be strikingly apparent when we come to consider the climatic character of a particular country.

Before proceeding to special examination of the climate of the United States, it will be well to take a glance at the general laws of climate, as illustrative of their harmony, throughout the globe. It is an important general law in reference to both continents, that a striking analogy exists, on the one hand, in the climatic features of the western coasts, and, on the other hand, in those of the eastern shores. Thus, in tracing the same isothermal line around the northern hemisphere beyond the tropic, it presents on the east side of both continents, concave, and on the west side, convex summits. Following the mean annual temperature of 55°40' Fahr. around the whole globe, we find it passes on the eastern coast of the Old World near Pekin, and on the eastern coast of the New World at Philadelphia, both places being in the latitude of about 40°; and on the western coasts, the same isothermal line passes in Europe near Bordeaux, and in America at Cape Foulweather, south of the mouth of the Columbia, both points being about six degrees farther south than the corresponding mean annual temperatures on the eastern coasts. On comparing the two systems, the concave and convex summits of the same isothermal line, "we find," says Baron Humboldt, "at New York, the summer of Rome and the winter of Copenhagen; and at Quebec, the summer of Paris and the winter of Petersburg. In China, at Pekin, for example, where the mean

temperature of the year is that of the coast of Brittany, the scorching heats of summer are greater than at Cairo, and the winters are as rigorous as at Upsal."

Connected with this branch of the subject, there are many interesting relations, the consideration of which is precluded by want of space. Reference may, however, be made to several points. It is only within the temperate zone, from about 30 to 60° of north latitude, that the year exhibits the grateful vicissitudes of the four seasons-the varied charms of spring and autumn, the tempered fires of summer, and the healthful rigors of winter. Wisdom desires not that "eternal spring" the want of which poets affect to deplore. At the equator there is no difference between the mean temperature of summer and winter, but it increases, as a general rule, with the latitude. In the United States, for example, the contrast in the seasons, from Florida to Canada, increases in proportion as the mean annual temperature decreases, a general law subject to modification on every parallel in accordance with the varieties in physical geography. This is strikingly illustrated in tracing the same isothermal line around the globe. Thus, in following the mean annual temperature of 51°, we find the mean temperature of winter in the mild climate of England as high as 38°; in Hungary, which is the interior of a continent, it sinks to 329; in China, in the austere climate of the eastern coast of a continent, it sinks still lower, being 24°; having reached the uniform climate of the western coast of America, it rises suddenly to 41°; and approaching the rigorous climate of our eastern shores, it is once more seen to sink, at Council Bluffs on the Missouri, as low as in China; and reaching our eastern shores at Fort Wolcott, where the temperature is modified by the ocean, the mean winter temperature again rises to 32°. In regard to the mean temperature of summer, the law is reversed, each climate having a summer temperature high in proportion as that of winter is low. In England, the mean temperature of summer is 63°; in Hungary, 70°: in China, 79; on the western coast of America, 65°; at Council Bluffs, 76°; and at Fort Wolcott, R. I., 69°. Hence it is demonstrated that

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the climate is most uniform on the western coasts of Europe and America, and least so on the eastern shores of Asia and America; that in Europe it grows more rigorous, in proportion as the eastern part is approached; and that in eastern America; the interior, remote from large bodies of water, is more austere than the region of the Atlantic. These results find a ready explanation in physical causes. Between our coast and the western coast of Europe, for example, the difference of climate is so marked, that Fort Sullivan, on the coast of Maine, notwithstanding it is more than 11° south of Edinburgh, Scotland, exhibits a mean annual temperature of 51° lower; and Bordeaux, France, which is parallel with Fort Sullivan, has an annual temperature of 15° higher. Consequently, we find that Europe is separated from the polar circle by an ocean, whilst eastern America stretches northward at least to the eighty-second degree of latitude. The former, intersected by seas which temper the climate, moderating alike the excess of heat and cold, may be considered a mere prolongation of the eastern continent; whilst the northern lands of the latter, elevated from three to five thousand feet, become a great reservoir of ice and snow, which diminish the temperature of adjoining regions. These results gave rise to the opinion that the old continent is warmer than the new, until more recent observations showed that the western coasts of both continents have a higher temperature than the eastern in corresponding latitudes.

The rationale of these laws finds an explanation in a grand natural phenomenon, which may be designated the great atmospheric circulation. As the accumulated heat of the sun in the tropical zone rarifies the air, it ascends into the higher regions of the atmosphere, and flows off towards the north and south; and, on the other hand, to maintain that atmospheric equilibrium which the barometer informs us always subsists throughout the globe, lower currents of heavier air from the northward and southward sweep into the tropical regions. These polar and tropical currents do not flow directly north and south. The former, as the velocity of every point of the earth's surface increases from the poles to the equator,

and as the particles of these currents cannot at once acquire a velocity equal to the continually accelerating velocity of the parts of the earth's surface over which they pass, will gradually seem to acquire a motion in an opposite direction to that of the rotation of the earth. Thus, the polar currents will assume in the northern hemisphere the character of a north-east, and in the southern, of a south-east wind, both becoming more easterly as the equator is approached, and constituting the Trade Winds. The winds without the tropics, on the other hand, have a prevailing direction from the west-a fact which affords a solution of the problem that in extra-tropical latitudes, countries lying to the eastward of seas or other great bodies of water, have milder climates than those situated on the eastern portions of a continent. As large bodies of water never become so cold in winter or so warm in summer as the earth, the winds that sweep from them have a constant tendency to establish an equilibrium of temperature. Landwinds, on the contrary, must necessarily bear with them the greater or less degree of cold induced by congelation, whilst in summer they will convey the accumulated heat absorbed by the earth; and thus is produced, in a great measure, those extremes of the seasons which characterize extra-tropical latitudes on the eastern coasts of continents.

In either hemisphere above the latitude of 30°, a westerly breeze of considerable force and regularity prevails. These winds cross the Atlantic from Newfoundland to Cornwall, and traverse the southern ocean from the Plata to the Cape of Good Hope, and thence to New Holland. Any wind which blows from the south of west, comes really from the equatorial region, and is therefore relatively warm. According to observations made by John Hamilton, during twenty-six voyages between Philadelphia and Liverpool, from 1799 to 1817, the winds were more than half the time from the west. Thus, out of 2029 days, the wind prevailed:

The same fact is established by the average period of the passage between these two points, the mean from west to east being 23 days, and from east to west 40 days.

There are, however, many other causes in operation, a few of which will be here noticed. Philadelphia and Pekin, each on the eastern coast of its respective continent, and nearly in latitude 40°, have the same mean annualtemperature; whilst on the western coast of the Old and the New World, the same annual temperature is found about the 48th parallel. Continuing this comparison, it will be seen that the climate of the New World, viewed in its general features, is, contrary to common opinion, more mild and uniform than that of the Old. As this is proved by thermometrical observations, the fallacy of the opinion which ascribes the mild climate of Europe to the influence of agricultural improvement, becomes at once apparent; for the region of Oregon, lying west of the Rocky Mountains, which continues in a state of nature, has a climate less contrasted than that of Europe in similar latitudes; and it is consequently, in a proportionate degree, milder than the climate of our own region, in which the labors of man, in a few ages, have almost wrought miracles, as well as that of the eastern coast of Asia, which has been under cultivation for several thousand years. Another cause of the higher temperature of Europe is, that the surface of the earth, between the 40th parallel and the equator, absorbs a large quantity of caloric, which is diffused by radiation into the atmosphere. Thus, Africa, as Malte-Brun observes, "like an immense furnace, distributes its heat to Arabia, to Turkey in Asia, and to Europe." On the contrary, the north-eastern extremity of Asia, which extends between the 60th and 70th parallels, and is bounded on the south by water, experiences extreme cold in corresponding latitudes.

Still another cause contributing to the same effect is the Gulf-Stream, which stretches across the Atlantic between Cape Hatteras and the Azores, forming, nearly in the middle of the northern Atlantic, a lake of warm water, which, according to Rennell, is not inferior to the Mediterranean in extent. Whilst a cold polar stream sweeping immense masses of ice into lower latitudes, is directed upon the coast of