Yov. 4, 1875] 



NATURE 



II 



FA YE ON THE LAWS OF STORMS* 



Mechanical Identity of Waterspouts and Eddies. — The 

 estion then is reduced to this, viz., whether, when in the 

 iniddle of the most profound calm these destructive 

 waterspouts are [seen to appear, the form of which corre- 

 sponds so well to the eddies formed in streams of water, we 

 can point to any current by [which the plienomenon has 

 been originated. Now this is precisely what we intended 

 to set in strong relief in describing the general currents 

 of the atmosphere. The counter-trades clearly show that 

 there exist above our heads unmistakable currents of air 

 in motion. Without even recurring to considerations of 

 this nature, it is enough to cast our eyes over the heavens 

 on the appearance of a waterspout, in order to see by the 

 march of the clouds that in spite of the calm below, 

 powerful horizontal currents prevail aloft, which, as their 

 different parts cannot advance at the same rate, must 

 consequently give rise to whirling movements of a more 

 or less decided character. If one of these whirls meet 

 with the favourable circumstances so often seen in water- 

 streams, it will be regularly developed by taking the form 

 which analysis assigns to it ; by its downward movement 

 it will even penetrate through into the calm strata, the 

 resistance of which will gradually alter its form and its 

 course, and end by reaching the ground. This simple 

 notion of the mechanical identity of gyrations, whether of 

 liquids or gases, furnishes at once the explanation of phe- 

 nomena which meteorologists have laboured to find in an 

 entirely different range of ideas. Waterspouts contain 

 powerful forces in action, because they draw the force 

 from a medium above, where it is in abundance ; they 

 march onwards because they follow the current which 

 originates them ; at their base they are slightly curved, 

 not forwards but backwards, because the comparatively 

 still medium they traverse offers a certain amount of re- 

 sistance ; they act nevertheless in the same manner both 

 on the ground and over water, whatever be the curvature 

 of the conical tube which descends from the clouds, 

 because this curvature never interferes with the direction 

 of the axis of rotation of each spiral, but only with the 

 succession of these spirals in space, Sic. 



This identity of waterspouts in air and of eddies in 

 water, which is so complete in a mechanical and geome- 

 trical point of view, is no longer to be looked at altogether 

 from the physical standpoint, on account of the differences 

 which in this respect exist between water and gas. In- 

 deed, the temperature of a stream of water is almost the 

 same at all depths ; in the air, on the contrary, heat 

 decreases markedly as we rise to the higher strata. 

 Further, the moisture of the air is liable to be condensed 

 for a fall of temperature often very slight. Thence the 

 cold air of the high regions, drawn gradually downward 

 by the whirling movement into the low and moist strata, 

 generates a thin mist all round the waterspout. This 

 mist serves as an outer envelope or sheath, the form 

 of which is more or less sharply marked, being ren- 

 dered visible by its opacity. There is no doubt that the 

 air in its descent is subjected to an increasing pressure 

 and gradually rises in temperature ; but it is lower than 

 the temperature of the surrounding air, and it is enough 

 if it falls to the dew-point of the general mass of air sur- 

 rounding it in order that the nebulous sheath may be 

 immediately produced. If the difference of the two tem- 

 peratures is insufficient, or if the humidity is too low in 

 any particular stratum, the misty sheath will not be 

 formed, and the waterspout will in part be invisible. 

 None the less, however, will it be there, though it seem 

 cut in two, or appear only in its upper part in a truncated 

 form. This is the appearance so often presented by 

 waterspouts at their commencement, when the upper and 

 lower portions are seen, but not the intermediate portion. 

 Soon these detached portions meet, the outside sheath 



* Concluded from vol, xii. p. 538. 



completing itself as the stratum traversed by it becomes 

 slightly more humid, or as the air whirled more rapidly 

 downwards by the waterspout becomes slightly colder 

 than the air it meets. 



Just as happens in the case of the water which gradually 

 descends down an eddy in its whirling course, the air, 

 which gradually descends with a violent whirling motion 

 down the waterspout, escapes from it on coming into con- 

 tact with the ground, and thereafter rises again in an irre- 

 gular manner outside the waterspout. But the volume 

 of air which enters into ordinary waterspouts is far from 

 being sufficient to give rise, at a distance, to a wind 

 of any appreciable force ; it is only in proximity to and 

 immediately around the base of the waterspout, where 

 this irregular upward movement of the air manifests 

 itself by the ascent of the dust or spray already raised by 

 the lower spires of the meteor. The base of the water- 

 spout is then enveloped in a sort of confused cloud cease- 

 lessly renewed, unless the end of the waterspout ceases to 

 reach down to the ground. This is especially the case 

 when a waterspout suddenly meets a valley in its course ; 

 its lower end goes on lengthening, and with little delay is 

 again joined with the ground ; but if the movement of 

 translation is too rapid, it will not resume its destructive 

 work till it has cleared the valley and gained the opposite 

 higher ground. Thus in gases as in liquids, whirling 

 movements observe exactly the same laws. The idea is 

 simple and clear : let us, then, without hesitation, put it 

 in place of that of an aerial column with boundaries 

 formed doubtless of misty vapour, but really treated as 

 solid and impenetrable like the crystal spheres of anti- 

 quity,'through|which the cloud draws up the water of the 

 sea, trees, and other objects ; or, to put it differently, 

 through which a centripetal updraught violently draws 

 skywards sea-water, trees, &c. In accordance to our idea, 

 all becomes clear and simple in the history of waterspouts ; 

 with prejudice, on the other hand, all remains astounding, 

 incomprehensible, and contrary to the simplest notions of 

 mechanics. There, are, however, two points of detail yet 

 to be discussed : we have to return to the appearances 

 from which eye-witnesses have drawn such remarkable 

 conclusions, and to the part played by electricity, a force 

 which meteorologists, till quite recently, were always so 

 ready to resort to in the explanation of phenomena. 



As the purely physical appearances of waterspouts 

 differ widely from each other, some have failed to observe 

 the slightest trace of an internal movement ; others have 

 attributed to them a descending movement without rota- 

 tion ; and lastly, others, and these the most numerous 

 attribute to them a whirling ascending movement. A 

 little reflection easily explains these contradictions. What 

 is seen and what is related by eye-witnesses whose im- 

 pressions are vitiated by old-standing prejudice, has no 

 reference to the waterspout itself, which, like air, is trans- 

 parent and invisible, but to its external envelope of mist, 

 which is more or less opaque. The envelope is exterior, 

 we repeat ; it does not therefore partake in the internal 

 gyrations, which, moreover, are too rapid to be visible. 

 Only the surrounding air which is brought into contact 

 with the waterspout is rapidly drawn from some distance 

 by lateral communication with the whirling movement, 

 the result of which is a sort of whirling or spiroidal agita- 

 tion in the outside sheath of the waterspout. The 

 degree in which movements of this sort favour illusion is 

 well known. It is thus that the slight movements of the 

 cilia of rotifers have the appearance of a rapidly revolving 

 wheel, and the simple rotation of a spirally-cut cylinder of 

 glass produces the impression of a flowing stream of water. 

 Further, the air which is thrown out at the base rises 

 again outside the waterspout. The aqueous vapour im- 

 perfectly condensed in the outside of the sheath has itself 

 an ascending tendency sufficient to raise some of the 

 small cloudlets of mist found there. Here are the 

 real movements, complex and changing, but slow enough 



