They would be inclined, but in two different ways: the lower ones would be inclined toward the warm part, while in the upper layers the inclination would be the reverse. A full circuit of the lighter liquids flowing one way on the surface, and of heavier liquids flowing the other way on the bottom, would thus be established. The same would happen in our atmosphere with the lighter warm currents and the heavier cold currents if the earth had no rotation on its axis. But it rotates—the solid globe as well as its gaseous envelope—and this modifies the whole circulation. The air which flows from the equator to the poles maintains, not its velocity of rotation, as has been hitherto taught, but its energy of rotation, which means that it obeys the law of preservation of areas; therefore, when it is transported from the equator to a higher latitude it is endowed (in the northern hemisphere) with a much greater easterly velocity than if it simply maintained its speed of rotation. On the other side, the air which is flowing from the higher latitudes toward the equator also obeys the same law and acquires a westward velocity, but much smaller than the eastward velocity of the former; this is why the west winds have such a preponderance in our latitudes.[1] Moreover, in virtue of the centrifugal force, all masses of air moving in any direction—not only north or south, but also due west or east—are also deflected to the right in the northern hemisphere, and to the left in the southern hemisphere,[2] Consequently the air flows in great spirals toward the poles, both in the upper strata of the atmosphere and on the earth's surface beyond the thirtieth degree of latitude; while the return current blows at nearly right angles to the above spirals, in the middle strata as also on the earth's surface, in a zone comprised between the parallels 30° north and 30° south.[3]
Such are, very briefly stated, the leading features of the theory which Ferrol laboriously worked out during the last thirty years, submitting all its parts to the test of both observation and mathematical analysis. By the end of his life (he died in 1891) he embodied his theory in a well-written and suggestive popular work,
- ↑ Full tables giving the eastward (or westward) velocities for each latitude, under the two different hypotheses, have been calculated for the Meteorologische Zeitung, 1890, pp. 399 and 420.
- ↑ Ferrel seems not to have been aware that the same had been demonstrated by R. Lenz for rivers (about the year 1870), in a discussion of Baer's law, applied to the Amu Kiver, in the Mémoires of the St. Petersburg Academy.
- ↑ William Ferrel, A Popular Treatise on Winds, comprising the General Motion of the Atmosphere, Monsoons, Cyclones, Tornadoes, Waterspouts, Hailstorms, etc. New York: Wiley, 1889. See also analysis of it by W. M. Davis (in Science, xv, p. 142; translated in Meteorologische Zeitung, 1800; Literaturbericht, p. 41), who gave the best diagram of circulation according to Ferrel's theory, and by H. F. Blanford in Nature, xli, 124. A full bibliography of Ferrel's works was given after his death in the American Meteorological Journal, October, 1891.
