394 THE POPULAR SCIENCE MONTHLY. 



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 rota- 

 tion 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.* 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 de- 

 flected to the right in the northern hemisphere, and to the left in 

 the southern hemisphere.! 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. J 



Such are, very briefly stated, the leading features of the theory 

 which Ferrel laboriously worked out during the last thirty years, 

 submitting all its parts to the test of both observation and mathe- 

 matical analysis. By the end of his life (he died in 1891) he em- 

 bodied 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. 



f Ferrel seems not to have been aware that the same bad been demonstrated by R. Lenz 

 for rivers (about the year 1870), in a discussion of Baer's law, applied to the Amu River, in 

 the Memoires 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, 1890; Literaturbericht, p. 41), who gave the best diagram of cir- 

 culation 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. 



