Page:The American Cyclopædia (1879) Volume IV.djvu/692

680 CLIMATE rally lead to an examination of the physical formation of the earth, as a cause of this great discrepancy in climate in the same parallels of latitude. According to Malte-Brun, climate is the assemblage of all those external physical and natural circumstances connected with each particular locality, which have a bear- ing upon the modifications of its tempera- ture. This definition, adopted by M. Virey in the Dictionnaire des sciences medicales, and by M. Foissac in his work entitled De ^influence des climate sur Vhomme, is sus- ceptible of embracing all those phenomena involved in the complex study of physical climate, which are so varied in character, and frequently so little allied to each other, as to forbid their exact and rigorous classification. The actual climates of the earth, as practi- cally experienced by its living forms, animal and vegetable, are greatly diversified and highly irregular. The averages or fixed quan- tities for different latitudes and localities are widely variable, and the non-periodic range of the leading conditions is even more extreme. The causes of this marked exception to the otherwise well defined character of all move- ments of matter, and the reason why this de- partment of physics cannot, in the present state of knowledge at least, and probably not at any time, be the subject of precise analysis, may be seen from the following general statement. It is already admitted as certain that the various climates cannot be defined by any astronomical conditions, or by .the form and movements of the earth relatively to the sun. Yet the heat received from the sun is the fundamental con- dition and controlling cause in all cases, and it acts both directly and through the secondary agencies of aeriform fluids and all mobile forms of matter. That is, the heat of the sun's rays is one part of the supply, as those rays fall di- rect on the surface, while air, atmospheric va- por, and water are also powerful agents in re- ceiving and diffusing heat, bringing the direct product of the sun's power, as received in the tropics, far northward to soften the climates which cannot receive sufficient heat directly. The power of this tropical heat is enormous. Not only are animal and vegetable forms stim- ulated by it to their largest growth, but the whole inorganic mass capable of motion is put in active circulation. By this heat the air is rarefied and given rapidity of circulation, at the same time that vast volumes of water are converted into vapor, capable of absorp- tion and diffusion almost throughout the en- tire aerial mass. This system of air and water circulation causes, as one general result, tbe evaporation and subsequent precipitation in rain of a sheet of water that may be stated as being ten feet deep at the equator, six feet deep on the average in the tropics, three feet deep at the 45th parallel, and one foot deep or less at the Arctic circle. This vapor, being raised by heat, is precipitated in rain as the heat is dis- sipated ; and this absorption and diffusion are also concurrent with a systematic movement of the aeriform mass, a movement which in the tropics is against the rotary movement of the solid mass of the earth, and probably is due in part to simple retardation of these fluid vol- umes, relatively to this solid portion. An in- ward and upward movement within the trop- ics, consequent upon this heat and saturation, originates the great system of atmospheric cir- culation, and distributes to the temperate lati- tudes most of the moisture there deposited. A surplus of heat is also so diffused, and through these joint agencies the actual climates of a large portion of the earth's surface are estab- lished. And as the middle or temperate lati- tudes are in the direct path of the returning atmospheric current, the more striking general phenomena these climates present are easily explained in this connection. In passing northward and descending to the surface, these volumes of heated and saturated air lose their surplus of heat and most of their contained moisture ; and in moving from west to east in these latitudes this circulation brings mild tem- peratures and local humidity from any ocean surface traversed ; hence the profuse rains and moderate heats of the western coasts of both continents. In the interior, the same circula- tion continuing, there are alternations of both conditions ; the heat being greater in the sea- son of greatest heat and less in the cold season than on the coasts, and the rains being alter- nately profuse and deficient. The degree of these alternations is increased until the imme- diate eastern boundary of the continents is reached, when a limited and local maritime influence appears to neutralize the agencies prevailing on the interior surface. Climates distinctly maritime, or directly controlled by adjacent water surfaces, exist in but few posi- tions on the eastern borders of the continents. At Norfolk, Newport, Nova Scotia, and New- foundland on the Atlantic coast, there are quite decisive maritime influences particularly at or near Newport, and over most of the Nova Scotian peninsula ; but these are due to unu- sual exposure, and close proximity to the Gulf stream. Next to the atmospheric circulation in its influence on the actual distribution of heat is the movement of great sea currents. These currents are caused by the same tropical excess of heat, with its consequent rarefaction of the fluid mass of the equatorial latitudes. A movement westward within the tropics is arrested by the continental masses of both America and Asia, and deflected in the gulf of Mexico and on the coast of China, forming the Gulf stream of the Atlantic and the Kuro Siwo of Japan and the Pacific. These great currents move northward on the surface of vast seas, diffusing their excess of heat, and greatly modifying the climate of the land sur- faces within their reach. This agency is espe- cially powerful on the western coast and isl- ands of Europe, deflecting the atmospheric circulation itself, charging the air with hu-