590 



NA TURE 



[April 23, 1896 



The existence of whirlwinds with horizontal axes in hail- 

 storms, particularly in that of May 20, 1893, at Pittsburg, has 

 been observed by an American meteorologist, Mr. Frank W. 

 Very, and has furnished him with a very ingenious explanation 

 of the formation of hail. Indeed, .such a whirlwind (if it has 

 sufficient dimensions) takes the hot and damp air of the surface 

 of the ground to high and cold regions ; the vapour condenses, 

 freezes, and crystals of ice are brought into the gyratory move- 

 ment ; they ascend and descend alternately, following the 

 spirals of the whirlwind, and increase at every passage in the 

 inferior regions, which are charged with humidity. This 

 explanation accounts for all the peculiarities which are observed 

 in a fall of hail : zoned structure, very low temperature ; special 

 sound before the fall ; electric manifestations which accompany 

 them ; for a whirlwind of hail is a veritable influence electric 

 machine, a sort of replenisher. 



Artificial Reproduction of Natural Gyratory Phenomena. — The 

 phenomena produced by the rapid rotation of the air are altogether 

 unexpected in consequence of the singularity of forces put in play. 

 The ordinary laws of mechanics, to which daily experience has 

 accustomed us, seem entirely different to those which the cyclonic 

 movements seem to obey ; and this must not astonish us. 

 We have reduced mechanics to its simplest elements ; the 



We will, therefore, not endeavour to analyse the forces put 

 play in the gyratory movements of the air. I will limit myself 

 to repeating before you some of the beautiful experiments of 

 M. Ch. Weyher, who has been good enough to come himself 

 to help me arrange the apparatus now before you. 



Here is a sphere composed of ten circular paddles, put in 

 rapid motion round axis A B (Fig. 4) ; the air caught in the 

 rotation produces a general whirlwind movement, symmetric in 

 relation to the plane of the equator. On all sides the air isi 

 sucked in by the revolving sphere, which may be seen by the 

 effect on smoke or pieces of paper brought near it. This air is ex- 

 pelled from the equatorial circumference, and only in the almost 

 mathematical plane of this circumference ; in fact, look at these 

 pieces of paper which keep themselves concentrically to the 

 equator, following an arrangement which reminds us of Saturn .s 

 ring. The tension of the paper and its vibrations show that it 

 is the repulsion of the equatorial outflow which maintains them. 



It might be concluded from this, that the revolving sphtre 

 could only produce equatorial repulsions ; but the complexity of 

 the turbulent streams baffles the most evident anticipations. If 

 a light balloon be approached a little distance from the sphere, 

 it is immediately alt' acted, and begins to revolve rapidly round 

 the sphere in the equatoiial pinne : if a second < r lliird be let 



I 



self* 



Fig. 4. — Artificial reproduction of the gyratory natural phenomena. 



Fig. 5. 



material point, the constant force, the rectilinear movement : 

 thanks to these simplifications, we have been able to understand 

 the movement of spherical projectiles, that of a pendulum, the 

 rotation of a fly-wheel, &c. But as soon as the solid body 

 becomes complex as to its form, when the movement which it 

 may take has at the same time a translation and a rotation, our 

 imagination represents it badly ; if to this complication of 

 form we add the resistance of the surrounding medium, then 

 we have no idea of the probable resulting effect ; for example, 

 the boomerang. As to the movements of fluids, they are so 

 difficult for us to foresee, that we receive fresh surprises every 

 time we move a vessel of water ; as soon as the mass of water 

 is at all considerable, the tumultuous movements, which we 

 unwillingly cause, always produce some awkwardness. 



We understand then how impossible it is for us to anticipate 

 the atmospheric movements, of which the mass is so immense, 

 for each cubic metre weighs 1,300 grs. ; if the energy 

 expended in setting in movement such masses is considerable, | 

 inversely the stability of the system is enormous, since we have 

 to wait for the dissipation of this energy by the passive 

 resistances, almost always reduced to friction on the earth's I 

 surface. i 



loose in the same way, they v\ill follow it at varied velocity, and 

 represent satellites ; the planetary configuration is complete. 



This paradox of a repulsion transformed into attraction Ijy a 

 change of form of the presented body, is easily solved by con- 

 sidering the resultant of aspiring and repelling actions on the 

 surface of the moving body. On the greatest angular space 

 round the revolving sphere it is the whirlwind attraction which 

 dominates. This is easily proved by placing underneath this 

 sphere a basin full of hot water ; if the atmosphere of the 

 room is quiet, little by little the vapour will be seen to collect in 

 a whirl from the surface of the water to the revolving sphere 

 (Fig. 5). This is the imitation of a water-spout. The importance 

 of this phenomenon has led M. Weyher to reproduce it in 

 a more striking way, and by bringing into play a much more 

 considerable quantity of mechanical energies, thus recalling 

 better those which constitute this natural phenomenon. 



The excitement of the gyratory movement (which, in nature, 

 has its source in higher regions of the atmosphere) is produced 

 by a small mill, placed three metres above a reservoir of water 

 four metres in diameter (Fig. 6). When the small mill is made 

 to revolve (400 to 500 revolutions a minute), the aerial whirl- 

 wind sucks up little by little the surface of the water, which is 



NO. 1382, VOL. 53] 



