Local conditions within the continents also affect production and producing possibilities. The great elevations which we call mountains and the tablelands adjacent to them lift their producing surfaces into temperatures radically different from those lying alongside on a lower level. Certain areas in tropical America for example, which lie a mile or two miles above the ocean level, enjoy a temperate zone climate although distinctly within the area designated as the tropics, while the adjacent areas lying but a few feet above the ocean level swelter in the fiercest tropical heat and their respective products vary in like propor tion. On the other hand, certain land masses in the far interior of the continents are so distant from the ocean that they receive little if any rain, for, as is well known, all the water which the land receives is carried to it by the currents of air which have obtained their moisture from the evaporation of the ocean. Air which has been so charged in passing over the ocean begins to lose its moisture in the form of mists, rain or snow as it passes over the land and is forced upward, and so may lose practically all of its moisture before i reaching the far interior of the continent, espe cially if it crosses high mountain ranges in its passage. As a result, we have the great desert areas in central and southwest Asia, North Africa, in the western part of the United States, the interior of Australia and certain sections of South America. Thus certain areas which lie within the tropics may have a tem perate zone climate and certain other countries in the temperate zone or tropics may be abso lutely unproductive by lack of moisture, which. is plentiful in the same latitude a few hun dred miles away.
Air currents and the currents of water which they have formed in the ocean have also a marked influence upon the productive con ditions of the land, especially in the vicinity of the ocean. The ((trade winds)) so-called or air currents moving from the north and south toward the equator to fill the partial vacuum caused by the upward movement of the in tensely heated air of that region blow more and more toward the west as they approach the equator, by reason of the fact that the surface of the earth at that point is traveling toward the east more rapidly than that part over which they passed in their movement from the north or south. This continuous flow of air toward the west during untold ages has set up currents of water in the ocean over which it passes, which also move toward the west in the vicinity of the equator, and when these currents of water come in contact with the land of the continent, they are forced to the north or south as circumstances may dic tate, and carrying the great masses of water heated by their long passage through the tropics affect, to a greater or less degree, the climate of the land masses with which they come in contact. The great northern equatorial cur rent which flows west across the Pacific Ocean from the Panama Canal to the Philippine Islands and the Asiatic coast is deflected northward along the eastern frontage of Asia and thence eastward to the northwest coast of North America, carrying some of the heat which it received in the tropics and materially modifying the climate of Japan and also that of the west coast of North America from Alaska to the southern part of the United States. Not only does it modify their climatic
conditions but also gives to those areas much greater supplies of moisture than they would otherwise receive. The similar currents of air and water crossing the Atlantic Ocean just above the equator force the current of warm water through the Caribbean Sea and the Gulf of Mexico and thence northward along the east coast of the United States and across the north Atlantic forming the aGulf Stream,a which carries warmth and moisture to north west Europe. These currents of air and water from the vicinity of the equator affect climatic conditions and the productive powers of cer tain sections of the continents with which they come in contact and prove an important factor in their production -and contribution to com merce.
By these various conditions, the temperate and tropical climates, the differences of eleva tion, and the modification of climate due to air and ocean currents, nature has greatly diversified the production of the earth's sur face, while she has also by other processes distributed beneath the surface the minerals which man has learned to utilize in the manu facturing industries and in transportation of the various natural products now so freely interchanged the world over. Within the last century man has learned to apply the power of steam and that of the waterfall in the form of electricity to the transportation of the great natural products of these various sections of the globe, and also to utilize these powers in transforming the natural products into the form required to make them useful to man. One hundred years ago there was not in all the world a steamship on the ocean or a mile of railway or telegraph on land. Now steam ships seek their landing place at the frontage of every continent, and 700,000 miles of railway bring the products of the interior to the water's edge or receive that which the steamship has brought across the ocean and distribute it to the interior. One hundred years ago the world had less than 800,000,000 people; now it has over 1,600,000,000 with vastly greater produc ing and consuming power per capita than those of a century ago, and their vastly greater com merce, $40,000,000,000 per annum at the present time against $4,500,000,000 a century ago. Thus geography and geographic conditions, coupled with the power of transportation between the geographic sections and areas produce com merce, and in studying commerce we must study the geographic conditions which bring it into existence.