Page:The American Cyclopædia (1879) Volume IV.djvu/693

 CLIMATE 681 raidity, and rendering the general climate ex- ceptional in the highest degree. Also in the Pacific ocean the agency of the Japan current is very great in bearing heat and moisture to higher latitudes than they otherwise would go. The western coast of America, from San Fran- cisco to Behring strait, is bathed in this warm and softening influence, receiving much more of both heat and its attendant conditions than the atmospheric circulation alone would bring. In the southern hemisphere a similar but less powerful movement of the sea currents attends the atmospheric circulation, with consequences generally similar in modifying the climate of the land surfaces, but less striking and impor- tant. There are also in both hemispheres re- turning cold currents of much local impor- tance, affecting the climate of certain tracts on the immediate coasts. The most important of these is the cold deep-sea current of the Pacific ocean, which, returning southeastward from the northern areas of that sea beneath a warm surface current, impinges on the W. coast of North America, and rises to the surface from the mouth of the Columbia river to Monterey, giving rise to the extraordinary cold day winds of summer at San Francisco, and over along line of that coast. This very striking fact finds no solution in any general theory of distribution of heat, either as received direct from the sun or as modified by atmospheric circulation, with its attendant standards of maritime and con- tinental influences. It is due solely to the aqueous circulation, in a phase unknown to the eastern continent. As the result of this great diversity of controlling influences, there can be no determination of the leading condi- tions of climate otherwise than by observation. Even with certain standards for the equatorial belt and for the poles, as has sometimes been assumed, there can be no formula for inter- polating the measures of heat or of humidity for the intervening latitudes ; and observation must be continued for a period sufficient to cover* the average of non-periodic variations above and below a true mean. These non- periodic changes are very great, and both in their degree and origin are in the present state of knowledge to a great extent inexplicable. An average for ten years is, however, nearly sufficient to give a true mean or fixed quantity in any of the conditions : the temperature, the fall of rain, and the date of significant changes, as the freezing or opening of rivers, the open- ing and closing of the season for growth of plants, &c. Observation of the leading condi- tions of climate has made great progress within the past half century, and a remarkable degree of continuous and faithful attention to the record of such observations has been shown by citizens and public institutions in the United States since 1820, while valuable and almost continuous series exist for some localities for a long time previous to that date, the earliest having been begun in 1738. In Europe the records of instrumental observation are but lit- tle more extended, the earliest being about 1720. Anterior to the discovery of instru- ments suited to measure the degree of heat, the weight of the atmosphere, &c., the tone of speculation and discussion of climatologi- cal laws was very loose, and full of highly imaginative views and conjectures. With the invention of the thermometer and barometer a new impetus was given to the study of clima- tological physics, and a vigorous attempt was made to apply positive measurements and to dis- cover absolute laws. But it soon became ap- parent that the range of non-periodic phenom- ena was too great, and that the differences were too striking which were disclosed in the comparison of localities which should, so far as could be seen, very nearly -agree, to admit of calculation on recognized physical laws. A long period then elapsed in mere waiting for the requisite basis of observed facts on which a legitimate and safe induction could be found- ed, and this period had not closed when Hum- boldt published his striking and effective gen- eralizations, a system of illustration of the dis- tribution of heat by isothermal lines. Time has not yet been afforded for the accumulation of observations of sufficiently extended periods over more than a small portion of the earth's surface, and it is not possible yet to fix the dis- tribution of heat with accuracy over any con- siderable part of the southern hemisphere. But the general outline of a positive climatology put forth in Humboldt's treatise has been greatly extended and filled up by subsequent labors, until it has attained a fixed position among sci- entific determinations. The leading condition determined is the mean temperature ; next to that is the average rainfall, and next the result- ant or average direction of the winds. The barometric measurements have been equally well established; and also the range or mea- surement of departure of each of these condi- tions from the mean, both periodic and non- periodic. The hygrometric condition, or the proportion of suspended moisture in the air, has been less generally observed, but it is rec- ognized as a necessity in the proper definition of climates. For all these the periods of observa- tion in many parts of Europe and of the United States are ample, covering, as has already been said, more than a century at a few sta- tions. But the necessity for a wide distribu- tion of observed positions delays the most im- portant of the uses to which these observations may be put. We turn, therefore, from the dis- cussion of the ultimate laws concerning the principal conditions of climate, to the consid- eration of the remarkable facts of actual cli- mate in the countries where determinations are recent. In the eastern part of the United States the facts have been long known, but in the interior and the west much is new. While the refrigeration originating with the mere pres- ence of great continental areas is great, and de- velops a continual series of violent alternations of temperature over much of the surface in the