12 



NATURE 



{Nov. 4, 1875 



to be visible. The illusion of the observer lies in attri- 

 buting to the interior of the waterspout the movements 

 which really take place round and outside its exterior 

 margin. 



The part plajed by electricity has been thus stated by 

 Peltier, who supposed he had detected traces of this force 

 in the well-known waterspout of Monville. The sheath 

 of vapour is in some sort a continuation of the electrically- 

 charged clouds ; it forms a long conductor of about eight 

 hundred feet between the clouds and the ground, a con- 

 ductor doubtless very imperfect, but on a great scale, and 

 capable of affording to some extent a passage to the 

 electricity. It is, however, far from being comparable 

 with the destructive characteristics of the thunderbolt. 

 The way in which trees overturned by whirlwinds are 

 sometimes broken up has been recognised as resembling 

 more or less that of trees struck by lightning and shat- 

 tered into splinters ; but this cffiect is only the result of 

 the violent torsion exerted by the gyratory movement of 

 the whirlwind, and not of the sudden passage of an electri- 

 cal current. Men and animals have often been caught by 

 whirlwinds and injured, without ever experiencing the 

 least electrical shock. 



Thus the essential characteristic of these remarkable 

 movements which produce waterspouts or great tornadoes 

 is a circular gyration, the spirals being slightly inclined to 

 the horizon. Wherever you make a section of it, you 

 only find there concentric circles with the radii always 

 converging towards a centre. In representing them geo- 

 metrically you need not hesitate between the circular 

 diagrams of Reid, Redfield, and Piddington, and the dia- 

 grams with converging rays of some learned meteoro- 

 logists, the victims of a hypothesis and old prejudice. 

 The former diagrams reproduce the mechanical pheno- 

 menon in its essentials ; the latter answer to a mere 

 illusion which a little reflection should ages since have 

 exploded. 



Extension of this Identity to Cyclones. — The last step 

 only remains to extend these conclusions to great tor- 

 nadoes, that is, typhoons, and lastly to cyclones, which 

 often overspread a vast extent of territory. It is one of 

 the characteristic properties of the eddies generated in 

 currents of water, that they are formed on every scale, 

 even the largest, without undergoing any essential change. 

 Eddies may be a few inches in diameter, a few yards, a 

 few furlongs, or even of still larger dimensions ; it is the 

 breadth of the currents where they are generated which 

 alone limits their size. In the ocean there are gyrations 

 on a still vaster scale, or even on a scale altogether 

 colossal, such as the vast currents of the Atlantic which 

 circle round the calm region of the Sargasso Sea. The 

 sun presents the phenomena of whirling movements still 

 better defined and of all dimensions, from large openings 

 equalling our cyclones, even to those large spots which are 

 five or six times greater than the earth itself. In like 

 manner, in the whirling movements of our atmosphere 

 are found small, short-lived eddies of a few feet in dia- 

 meter, whirlwinds and waterspouts, which last longer, 

 from 10 to 200 yards across, and tornadoes from 

 about \ X.0 1 5 mile in diameter. Beyond this the eye 

 cannot^ take in the forms of the whirhng columns ; 

 these receive another name, but in all essential points 

 they remain the same. When the dimensions are still 

 greater, the diameters measuring 300 miles and upwards, 

 they bear the name of hurricanes or cyclones ; but not- 

 withstanding this, their mechanism remains unchanged. 

 They are always gyratory, circular movements increasing 

 in velocity as they near the centre ; are generated in the 

 upper currents of the atmosphere, through the inequalities 

 of their velocities ; are propagated downwards through 

 the lower strata in spite of the calm or independently of 

 the winds which there prevail ; ply their destructive 

 energy when they reach the obstacle offered by the 

 groimd ; and follow in their march the upper currents, so 



that the track of their devastations marks out on the sur- 

 face of the globe the route of the viewless currents of the 

 upper regions of the atmosphere. 



There is, however, a difference between whirlwinds and 

 tornadoes on the one hand, and typhoons and hurricanes 

 on the other. As regards the former, note in the first 

 place, the upper portion (embouchure), which is a sort of 

 truncated cone inverted and very much widened out 

 above, and in the second place the descending column 

 which prolongs the meteor even to the ground. If the 

 atmosphere was a gaseous mass of air of indefinite height 

 like that of the sun, cyclones would always present these 

 two features. As regards cyclones, however, the ground 

 is very near in proportion to the extent of area they cover, 

 and is reached before they can be subjected to the pro- 

 longed contracting process seen in waterspouts and 

 whirlwinds. A cyclone is then a vast whirlwind, but 

 reduced by the obstacle offered by the ground, to 

 the upper part, or to what may be called the funnel- 

 shaped portion of the phenomenon strictly so called. 

 Thence, doubtless, the constant presence in the former of 

 a calm space about the centre, of which the analogue is 

 to be found only in the circling movements of the ocean 

 on their grandest scale ; and thence also certain impor- 

 tant peculiarities of cyclones to be more particularly 

 insisted on, after having examined the movements of 

 translation of these phenomena. 



Cotase of the Upper Trade Winds. — When the atten- 

 tion is directed to whirlwinds which appear most fre- 

 quently to be accidental phenomena of short duration 

 and merely superadded to other phenomena of a more 

 general character and much more lasting, it must be 

 allowed that the short lines marking out their course have 

 scarcely been studied from a geographical point of view. 

 These lines probably follow no simple law. In this 

 respect it is otherwise with cyclones ; their course 

 recurves, as we saw at the beginning of these articles, on 

 the globe in accordance with a particular law the con- 

 stancy of which Fig. 2 (vol. xii. p. 402) reveals at a glance. 

 From this chart, the upper currents, whence cyclones 

 derive their origin and mechanical power, do not proceed 

 directly from the equator to the poles. They are deflected 

 at the outset toward the west, then toward the east, thus 

 describing over the surface of the globe parabolic curves 

 whose apices lie somewhere within a few degrees of the 

 polar limits of the surface trade-winds. Clearly these 

 upper currents, which are true aerial rivers, ought to form 

 a part of the upper trades whose existence is assumed, 

 but their actual course is not directly known. If this 

 assumption be correct, then Fig. 2 presents at once the 

 projections of the double system of trades and counter- 

 trades over both hemispheres ; and it only remains to 

 explain the singular recurving course taken by the upper 

 trades. This explanation we shall attempt, though the 

 question lies a little out of our way. 



If the atmosphere were withdrawn from the influence 

 of the solar heat, it would remain in. equilibrium ; its suc- 

 cessive strata would arrange themselves according to sur- 

 faces of level, and would become part and parcel, so to 

 speak, of the solid globe itself ; at least it would, even as 

 regards the highest strata, exactly follow the earth's rota- 

 tion. The effect of the solar heat is constantly to disturb 

 this equilibrium, by the introduction of movements which 

 are the more curious inasmuch as they do not essentially 

 destroy the normal stratification of the strata of the atmo- 

 sphere. The air incumbent over the hemisphere actually 

 facing the sun is expanded in its lower strata, where the 

 opacity arising from dust floating in the air, and above 

 all the aqueous vapour, absorbs a large part of the heat- 

 rays of the sun. The intervention of this aqueous vapour 

 which ascends vertically from stratum to stratum, has in 

 a special manner the effect even of rendering the diurnal 

 variation of temperature perceptible at heights at which 

 it would not be felt if the air was dry. The maximum of 



