Page:The American Cyclopædia (1879) Volume XI.djvu/461

 METEOROLOGY 447 attempts to predict the weather, and especially storms, which are now daily made by the mete- orological offices of numerous civilized nations would of themselves seem to imply the exis- tence of a deductive science more or less de- veloped. This however is not necessarily the DIAGBAM XVII. Weather Eose for Eastern and Middle United States. case, for it has been found practicable, on the one hand, by means of the telegraph, to collect in a few hours material for compiling a dai- ly weather map for the whole of Europe or the United States, and, on the other hand, to apply to such maps the numerous generalizations that have been found to hold good for the respec- tive portions of the world ; a process which, repeated from day to day, reminds one of the methods adopted in astronomy for computing special perturbations. There seems to be no good reason for speaking disparagingly of me- teorology as a science, since, whether we study the stars, the atmosphere, atoms, or organic na- ture, we find ourselves everywhere confronted by an overwhelming mass of phenomena which are subject as yet only to an inductive philoso- phy. Owing to the infinite number of com- binations among the meteorological elements, no empirical rules can invariably lead to cor- rect predictions ; but the calculus of probabili- ties shows that over 50, and often 75 per cent. of our predictions should be well verified, a conclusion in harmony with actual experience throughout the world. But this percentage of verifications, we have reason to believe, is sensibly greater when the predictions are based not merely upon empirical rules, but equally upon a consideration of such general principles as must form the groundwork of the true de- ductive science. The foundations of the new meteorology are necessarily found in the sim- ple laws of mechanics. They have been con- sidered by several authors, but by none with so much completeness as in the work of Ferrel 550 VOL. XL 29 " On the Motions of Fluids and Solids, relative to the Earth's Surface " (Nashville, 1854, and New York, 1860). In this treatise the motions and figure of the atmosphere are first treated of, on the hypothesis that no resistance is offered by obstacles or by friction upon the earth's surface ; in a subsequent section the influence of such resistance is considered, on the hypothesis that a uniform coefficient of friction obtains over the whole earth's surface. Mr. Ferrel thus deduces the necessary existence of two belts of high barometric pressure, ex- tending entirely around the globe between the tropics and the parallels of 30 N. and S. re- spectively, and of a belt of low pressure at the equator, as well as regions of low barometer within the arctic and antarctic circles;' belts of calms near the equator, the tropics, and the polar circles are also deduced. The continents and oceans offer very unequal frictional resis- tances, which causes the equatorial belt of calms to lie on the average a little north of the true equator in the Atlantic and Indian oceans, but nearly on the equator in the Pacific ocean. Similarly, it can be shown that to the irregular distribution of the friction of the continents is partially due the breaking up of all these belts into segments lying respectively over the ocean and the continents. In general the motion of the earth's atmosphere is a consequence of the unequal heating by the sun of the equatorial and polar regions; the subsequent effort of nature to establish statical equilibrium relative to the earth's surface is that which gives rise to the phenomena of the winds. The unequal heating of the continents and oceans, due to their unequal specific heats and radiating pow- er, and to the annual vibration of the sun be- tween the tropics, is the principal cause of the breaking up of the belts of temperature, pres- sure, and winds into continental and oceanic areas. But whatever induces a local or general change in the density of the air disturbs its equilibrium and necessitates its motion; and the disturbing power next in importance to the solar heat is the diffusion of aqueous vapor, the density of which is only six tenths of that of dry air at the same pressure and tempera^ ture. It thus happens that the combined effect of friction, temperature, and moisture gives rise to the variable distribution of barometric pressure exhibited in the charts VII. and VIII., which show areas of high barometer existing in winter over the continents, but in summer over the oceans, and over the eastern half of each ocean, rather than over its central por- tions. The general distribution of barometric pressure having been thus deduced by Mr. Ferrel, he then considers the local disturbances known as cyclones, tornadoes, &c., all the phe- nomena of which are deducible from his ini- tial mechanical formulas (see " Bulletin of the Philosophical Society of Washington," June, 1874, and the "American Journal of Science," November, 1874), if we supplement these by the researches of Him, Peslin, and Keye.