Page:Popular Science Monthly Volume 33.djvu/838

818 and little has been done to grapple with them. What is known of these subjects is as yet almost exclusively empirical. Instrumental appliances are here far in advance of theories, and it is not to be disguised that great waste of labor too frequently results from an exaggerated refinement in observation, and subsequent numerical computation, which has no real value. The variations of the temperature, of the pressure, and of the motion of the air, and of the quantity of vapor it contains, give rise to the great series of phenomena which are included under the general term climate. Of these variations the primary causes are the action and reaction of the mechanical and chemical changes set up by the sun's heat as influenced by the earth's motion, terrestrial position, and the condition of its surface, as well as by fluctuations of the sun's heat itself, though of these last we know too little to do more than recognize their presence.

The conditions which determine at any place the greater or less degree and duration of direct exposure to solar radiation, and therefore the quantity of heat received there, are position in relation to latitude, combined with the diurnal and annual movements of the earth. The nature of the surface regulates the local accumulation of heat, by reason of the varying power of absorption or radiation possessed by different substances; while with elevation above the sea-level as the density of the air becomes less, the sensible temperature and the quantity of watery vapor are subject to corresponding change. The whole of the results thus produced, moreover, are modified by movements in the air consequent on atmospheric changes from place to place, or from time to time.

The inequalities of the earth's surface, which are insignificant when viewed in relation to the whole globe, are of the greatest importance in relation to the atmosphere. For, owing to the laws of elastic fluids, the great mass of the air and of the watery vapor it contains are concentrated very near the surface. One fourth of the air and one half of the vapor are found below 8,000 feet from the sea-level; one half of the air and nine tenths of the vapor are below 19,000 feet, which hardly exceeds the average elevation of the highest ranges of the Himalaya Mountains; while three fourths of the air and virtually the whole effective vapor lie below 30,000 feet, and therefore within the influence of the highest summits of those mountains. That portion of the atmosphere which is nearest the surface is manifestly the most likely to be acted upon by irregularities of relief, and by local variations in the power of absorbing or radiating heat or diffusing vapor. Hence it is certain that it is the movements of the lower strata of the atmosphere that chiefly affect all conditions of climate, though no doubt there are great movements in the upper regions to bring