436 METEOROLOGY
Dotted line* (or January ; full lines tor July. DIAGRAM I. Diurnal Variation of Tomperatur*. by the British admiralty in 1872. These lines depend on the immense mass of observations collected by that board, and by the labors of Dove, Maury, Buchan, and many others. The comparison of the isothermals for January and July shows at a glance the results of the com- bined influence of the solar altitude, the dis- tribution of continents, plateaus, oceans, &c. The more detailed analysis and comparison of these temperatures belongs to the subject of climatology. So far as our observations ex- tend, they show that the annual mean temper- atures of the points having the same latitude are about 2 higher in the northern than in the southern hemisphere. The average tem- perature of the whole earth, according to Dove, is greater in July than in January by about 8 F., although in July the earth is further from the sun than in January, in the ratio of 93 to 90. Besides the preceding jiormal and peri- odical fluctuations of temperature, as deduced from the average of many observations, the temperature of the air is subject to large non- periodic variations, which disturbances, prop- erly so called, are considered in connection with STOBMS, under that title. The more im- portant of these non-periodic variations ac- company the areas of moist and dry air, as these successively flow over the earth's surface. These disturbances in the periodical variation of temperature have their origin in terrestrial influences, and are the secondary reaction of the sun's rays themselves. 3. Winds and Cur- rents. If we neglect the slight amount of heat received from other sources, we may in gen- eral say that the winds are due to the combined influences of the rotation of the earth on its axis and the disturbances of the air, caused either by the introduction and subsequent con- densation of its vapor, or simply by the warm- ing of the lower strata of the atmosphere by the sun. The movement of the air is there- fore a question of mechanics, capable of solu- tion in just so far as mathematical analysis is able to fully take into account the combined influence of these primary forces, and the laws of pneumatics, friction, &c. This subject has been inductively investigated by Coffin, Bu- chan, Muhry, and Hann. The observations of the lower strata of the air are best obtained, as regards the directions of movement, by the self-recording wind vane, and as regards their velocity, by the self-recording anemometers. Apparatus by which these two anemometric elements are recorded continuously has in some form or other been adopted during the past few years by the government observers at a large number of stations throughout the world. The measurement of the pressure exerted against a flat surface by the wind is sometimes substituted for the direct measurement of the wind's velocity; but these two elements are only with difficulty and approximately to be compared between themselves. When obser- vers have no means of measuring force or ve- locity, an empirical scale is adopted, two forms of which are in common use, that of Admiral Beaufort and that of the Smithsonian institu- tion ; these are given in the following table. Owing to the great differences in the estimates
