:. 



May 4, 1882] 



NATURE 



When a somewhat weak current is passing between the knobs 

 of a Becker- Voss electro-induction machine, its passage can be 

 altogether stopped by simply blowing across the path of the 

 current. The handle is turned in vain ; and even when the 

 blowing has ceased. •» short time is required before the current is 

 able to pursue its old path. When the instrument has been 

 warmed, and the current becomes stronger, the blowing, although 

 now unable to stop the current altogether, drives it into irregu- 

 larly curved paths, which are determined by the force exerted. 

 I do not remember to have seen the experiment mentioned in 

 any book. It is as curious as it is simple. 



We now see why the air requires to be at rest for the weak 

 current to force a passage through it, and to keep that passage 

 open for the succeeding sparks to follow ; while the stronger 

 current leaps from point to point, as though in pursuit of the 

 warmed and opened passage which has been driven by the wind 

 out of its former position. Henry Bedford 



All Hallow's College, Dublin, April 15 



CYCLONES* 



SINCE it first became known that a considerable pro- 

 portion of the storms which visit this part of Europe 

 come from the middle and northern parts of North Ame- 

 rica, the meteorology of that country has been invested 

 with a peculiar and increasing interest for the inhabitants 

 of Western Europe, and though, according to Hoffmeyer, 

 the chance that a depression in the United States will 

 subsequently cause a storm somewhere in our own islands 

 is only one in four, it is a ratio quite substantial enough 

 to make us regard with attention warnings such as those 

 transmitted to us through the medium of the New York 

 Herald. 



While America is thus from her enormous size and 

 westerly position enabled to act the part of our weather 

 prophetess, she bids fair in addition to leave us far behind 

 in the more theoretical branches of weather-science, and 

 though to admit this may be somewhat wounding to 

 our national amour proprc, it is nevertheless an idea 

 which is openly entertained by some of our leading meteoro- 

 logists. For our comfort it may be reasonably ascribed, 

 in part at least, to our small size and unfavourable geo- 

 graphical position having afforded but little encourage- 

 ment to really able men to devote their attention to a 

 science whose operations are conducted on a scale com- 

 pared with which our area of observation is indeed 

 microscopic, so that until within quite recent times the 

 succession of fair and foul weather in these islands was 

 regarded merely as a series of irregular, eccentric, and 

 totally unpredictable changes. The work before us, 

 entitled " Methods and Results," by Prof. William 

 Ferrel, of the American Coast Survey, and prepared 

 tor the use of the coast pilot, forms the second part of a 

 series of meteorological researches undertaken by the 

 author, which comprise an elaborate theoretical investiga- 

 tion into the general and local mechanics of the atmo- 

 sphere. In Part I., which appeared in 1877, the general 

 motions of the atmosphere are mote particularly dealt 

 with, and conclusions are arrived at which have appeared 

 in part in the Mathematical Monthly for i860 and the 

 American Journal for November, 1874." 



In both these publications the author lays great stress 

 upon the important part played by the deflecting force 

 to the right of its path, to which a current of air is sub- 

 jected by virtue of the earth's rotation in whatever direc- 

 tion it may be blowing. This deflecting force is measured 

 by the acceleration 2 n cos yjr, where n represents the 

 angular velocity of terrestrial rotation, and i// is the co- 

 latitude (see Nature, vol. v. p. 384). 



With the assistance of this element he theoretically 

 deduces in Part I. the general motions of the atmosphere, 

 which agree with what is known from observation. He 



' " Methods and Results of Meteorological Researches for the use of the 

 Coast Pilot." Part II.— On Cyclones, Waterspouts, and Tornadoes. By 

 William Ferrel. (Washington, 1880.) 



a " Relation between the Barometric Gradient and the Velocity of the 

 Wind," by W. Ferrel, Assistant U.S. Coast Survey. 



also makes considerable use of this same principle, which 

 he was the first to enunciate correctly, when dealing 

 with the theory of cyclones in Part II. As we propose 

 just now to confine our attention to Part II., which treats 

 mainly of cyclones, we shall not refer to Part I. except 

 incidentally. Part II. is sub-divided into three chapters, 

 the first of which deals with the mechanical theory of 

 cyclones, and deductions therefrom. In Chapter II. the 

 results of the theory are compared with those of observa- 

 tion, and Chapter III. treats of tornadoes, hailstorms, and 

 waterspouts. The chief elements considered in the theory 

 of cyclones are (1) the earth's rotation, (2) the gyratory 

 velocity round the low centre, (3) the friction, (4) the 

 inertia, and (5) the temperature and humidity of the air. 



These elements are all discussed in turn, and many 

 important conclusions drawn from the resulting equations. 

 Some of these conclusions have already been either 

 directly deduced by the employment of analogous 

 methods, or inductively inferred from an examination 

 of data, by Guldberg and Mohn, Colding, Peslin, Sprung, 1 

 Clement Ley, Hildebrandsson, Meldrum, Loomis, and 

 Toynbee. Some however are quite new, especially those 

 which are derived from a consideration of the temperature 

 term. 



The general theory of the cyclone, according to Ferrel, 

 may be briefly stated thus : — 



If from any initial cause interchanging motions are set 

 up between the air in a certain district and another sur- 

 rounding it, the air in the first district tends to gyrate 

 round its centre by virtue of the deflective force of the 

 earth's rotation, and in the same direction as that of the 

 component of terrestrial rotation, which acts in the plane 

 of its horizon. In the northern hemisphere this would 

 mean gyration contrary to watch-hands, and in the 

 southern hemisphere gyration with watch-hands. In the 

 outer district the gyrations of the air, by the principle of 

 the preservation of areas (or moments), are contrary to 

 those of the interior district. These two systems of con- 

 trary gyrations tend to draw the air from the centre of 

 the inner district and the exterior limit of the outer 

 district, and heap it up in the place where the gyratory 

 velocity vanishes and changes sign, thus causing a maxi- 

 mum barometric pressure there, with corresponding 

 minima at the centre and outer limit respectively. 



In addition to this, when the gyrations have once com- 

 menced they give rise to a centrifugal force which tends 

 to drive the air still more from the centre of the inner 

 district, and so increase the barometric depression there; 

 but which in the outer district, partly owing to its dis- 

 tance from the centre, and partly to the small velocity of 

 the gyrations, has but little effect on the distribution of 

 pressure. The gyrations, especially near the centre and 

 exterior limit, would be very rapid, were it not for the 

 friction between the air and the earth's surface, which 

 retards the motion, but does not entirely prevent it, since, 

 as the author very pointedly remarks, "without some such 

 motion frictional resistance would not be brought into 

 action." So far we have only considered the gyratory 

 component of motion, and as in the imaginary case of no 

 friction, this would be the only kind of motion, the gyra- 

 tions might then be entirely circular. When, however, as 

 actually happens in the atmosphere, friction acts, a 

 radical component becomes necessary, since the deflecting 

 force is now partly employed in counteracting the fric- 

 tional resistance to the gyrations, and the magnitude of 

 this radial component (on which depends the inclination 

 of the wind to the isobar), varies cateris paribus directly 

 with the amount of friction. 2 As a result of the two 



1 " Die Tragheits-curven auf rotirenden Oberflachen," Zcitsc/iri/t filr 

 Mrieorolcgie, Band xv., January Heft, 1880. 



2 This result is best seen in the following expression for the angle of 



inclination of the wind to the isobar tan i - / cos . + s cos i \, where f 



is the coefficient of friction, s the velocity of the wind, and r the dis 

 from the low centre. 



