Page images
PDF
EPUB
[ocr errors][merged small][merged small]

of the North Atlantic is easily explained: the warm cur- Cause of the rents of the North Atlantic bear the heat of the Gulf of difference. Mexico and the tropical regions of America away from the eastern coast of that continent and give it to the western coast of Europe. Moreover, the prevailing winds of North America are from west to east, they are cold in winter and spring, and increase the difference in temperature caused by the action of the ocean currents.

[ocr errors]
[ocr errors]

3. Temperature of the United States. The conditions as to currents of air and water on the western coast of the United States are like those which prevail on the western

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small]

coast.

coast of Europe. A study of the temperature charts discloses the fact that the lines, as they approach the Pacific shore, spread out to a very marked extent. Thus the line Temperature of mean annual temperature of forty degrees reaches the of the Pacific Pacific near the sixtieth parallel, fifteen degrees farther north than it leaves the Atlantic. The case is even more marked as to the winter temperature, as the line of thirty degrees leaves western Europe and western America in nearly the same parallel. The summer temperatures of the

Temperature of the Mississippi basin and Atlantic coast.

Extremes of temperature.

Political re

sults of climatic conditions.

Pacific slope, however, are on the whole higher than those of western Europe.

In the interior of the two countries the difference is widely marked. A mountain barrier, the Cordilleran system, traverses the American continent.from north to south, and effectually prevents the winds of the Pacific slope from moderating the climate of the interior; but there is no such mountain barrier in western Europe. The winter lines, on leaving the Pacific coast, trend sharply southward, and the interior of the continent from the one hundredth meridian eastward has practically the same average temperatures as the Atlantic coast region in the same latitudes, but the extremes of temperature are even greater in the interior than on the eastern seaboard. The cold winds from the Cordilleras and from the frozen regions of Canada sweep down from the west and north, and produce great intensity of cold. On the other hand, in the summer time, hot winds frequently blow from the south and raise the temperature to a very high degree. The effects of these cold and warm "waves," as they are generally termed, are very great; the mercury is occasionally frozen in Wisconsin and Michigan, and even on the Gulf of Mexico the thermometer at Galvesron, Texas, has been known to fall fifty-four degrees in eighteen hours. The hot waves often produce extremely high temperatures, reaching one hundred degrees in Iowa and the neighboring states, and frequently attaining ninetytwo and over even on the shores of the Great Lakes, whose waters moderate the heats of summer.

The pinching together of the temperature lines on the eastern coast means that within the comparatively limited area stretching from the St. Lawrence to the Gulf of Mexico, agricultural conditions prevail which in the Old World are associated with regions extending from the North Cape to the Desert of Sahara. Each group of colonies and states has had its own industries, habits of living, and modes of thought. This fact has exerted its influence by dividing the people of the United States into sections and, to a

[ocr errors][merged small][merged small]

great extent, has determined the political history of the nation.

and of rainfall

4. Rainfall.— The temperature of a country determines Effects of its agricultural produce; thus Indian corn demands a good temperature degree of heat, while cotton will not thrive without a greater compared. intensity of heat. Corn, furthermore, is peculiarly sensitive to frosts, and will not ripen where they occur early in the autumn. Of equal importance is the amount and distribution of moisture. If the rainfall is excessive, the cotton plant makes a rank growth, or becomes so choked with weeds that it will not grow at all; but if it does not have abundant moisture, it will not thrive. Corn, on the other hand, absorbs moisture from the air, and will grow in seasons of drought, when other plants perish.

and distribu

Experience has shown that an annual rainfall of at least Importance twenty inches is essential to profitable agriculture. It is of amount true that a large number of important food plants, as corn, tion of will thrive on a less amount of moisture provided it is well moisture. distributed. Unfortunately, however, as the average rainfall decreases below twenty-five inches, the variation in the precipitation increases out of all proportion to the total amount. A diminution of five inches in any one year would make little difference in a region of thirty inches of average rainfall, but it would be fatal to the year's crops in a region of twenty inches of annual precipitation. Cultivation is only certain where the average rainfall is over twenty-five inches; although it is possible, but hazardous, in regions of twenty inches. Districts of from ten to twenty inches of rainfall are suited, as a rule, to grazing, but below ten inches vegetation of a valuable sort practically ceases. On the other hand, a rainfall much exceeding fifty inches produces a rank growth harmful to most plants suited to the soil and climate of the United States, although a few plants, as rice and the sugar cane, demand a large amount of moisture. Regions of over sixty inches of rainfall are suited only to these plants, but cotton will thrive when the rainfall is as much as fifty-five inches, and it demands as much as forty inches.

Distribution

United

States.

An examination of the map showing the average annual of rain in the rainfall of the United States shows us that the country east of the one hundredth meridian enjoys an abundant rainfall, which gradually increases as one proceeds to the east and southeast. Extensive failures of crops are rare, although they sometimes occur from a lack of rain during the growing season; failures from an excess of moisture are even

District of scanty rainfall. Shaler's

United States,

I, 17.

Rainfall at the Pacific coast.

Irrigation.

more rare.

West of the one hundredth meridian, especially in the summer, the rainfall rapidly decreases toward the west and southwest the temperature rising as the rainfall declines. In this way are produced large areas of hot and arid lands ill fitted, or not fitted at all, to cultivation, or even to grazing, unless artificial irrigation is resorted to, excepting narrow strips along some of the river valleys. The winters of this region are also longer and more severe than those of the country farther east.

This district of scanty rainfall extends to the coast ranges of the Pacific. The valley lowlands of the Pacific, with the exception of the valley of the Colorado, enjoy, not merely abundant moisture, but high temperature in the river bottoms, which rapidly diminish as the altitude increases. The result of these conditions is a marvelous cultivation; oranges and wheat grow within a comparatively short distance of one another. There are arid districts on the Pacific coast, and the rainfall is not evenly distributed; but the nearness of the rain-bearing mountains makes irrigation comparatively easy. The same system is also applicable to large regions east of the mountains, but the size of the districts requiring irrigation, and the distance of the water supply, make the carrying out of the enterprise exceedingly expensive. The returns from irrigated lands are usually large, and it is to be hoped that means will be found to develop the system in these warm arid regions. west of the one hundredth meridian.

5. Physical Formation of North America. - Students of history and geography have long been agreed that, within

§ 5] Physical Formation of North America

9

physical fac

tors on civil

ization.

certain limitations as to rainfall and temperature, the physi- Influence of cal formation of a country, the character of its soils, and the extent and variety of its mineral deposits exercise a decisive influence on the life of the people which inhabits it. To insure the best development, a country must offer easy access to the outer world. This is especially true of new regions, which require to be colonized from Europe. North America, especially that portion occupied by the eastern

Necessity of

ease of

access.

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small]

half of the United States, offers every inducement to the voyager to reach its shores. The low-lying Alleghany system, which protected the colonies from the savage aborigines of the interior, has proved to be easily surmountable by the railroad of modern times.

(1) Value of the Mississippi

Before the days of steam, the Mississippi basin was difficult of access. Many writers speak of the Mississippi and the St. Lawrence as the keys or gateways to the continent. In a limited military sense this is true; but only in a limited nists.

and St. Lawrence to colo

« PreviousContinue »