PHYSICAL PHENOMENA OF UPPER REGIONS OF ATMOSPHERE. 131 



structure, their very low temperature, the peculiar uoise before their 

 fall, the electric phenomena that accompany them; for a whirlwind of 

 hail is a true inductive electrical machine — a sort of replenisher. 



Artificial reproduction of natural gyratory phenomena. — The phe- 

 nomena produced by the rapid rotation of air are quite unexpected 

 because of the singular behavior of the forces set in play. The ordi- 

 nary laws of mechanics, familiar to us from our daily experience, appear 

 to be entirely different from those which the cyclonic movements seem 

 to obey. And this ought not to astonish us. We have reduced 

 mechanics to its most simple elements; a material particle, a constant 

 force, rectilinear motion. Thanks to these simplifications, we have well 

 mastered the motion of spherical projectiles, of a pendulum, of the 

 rotation of a fly wheel, etc. But as soon as the- solid body becomes 

 complex in form, whenever the movement that it may assume involves 

 both translation and rotation, our imagination does not well seize it. 

 If to the complication of form there is further added the resistance of 

 the surrounding medium, we then have no longer any idea of the 

 probable effect that will result. Witness the boomerang. As to the 

 movements, they are so difficult for us to foresee that we are always 

 surprised if we succeed in manipulating a vessel filled with water. As 

 soon as the mass of liquid becomes somewhat considerable the tumultu- 

 ous disturbance that we involuntarily cause in it always leads us to 

 commit some awkward act. 



You will see, then, how impossible it is that we should be able to 

 predict the movements of the atmosphere, whose mass is immense, each 

 cubic meter weighing 1,300 grams. If the energy expended in moving 

 such masses is considerable, inversely the stability of the controlling 

 forces is very great, since it must last until that energy is dissipated 

 by passive resistance, almost all of which is due to friction against the 

 earth's surface. 



We will not seek, therefore, to analyze the forces involved in the 

 gyratory movements of the air. I will confine myself to repeating before 

 you some of the beautiful experiments of M. Oh. Weyher, who has been 

 so kind as to come himself to assist me and to set up the apparatus that 

 l now place before you. 



You see here a sphere composed of 10 semicircular blades, made to 

 rotate rapidly around the axis AB (fig. 1, PI. I). The air attracted by 

 this rotation produces a general cyclonic movement symmetrical with 

 reference to the plane of the equator. The air is drawn toward the 

 revolving sphere from all sides, as may be shown by bringing near to 

 it smoke or bits of paper. This air is expelled along the equatorial 

 circumference, as you will see by these paper wreaths, which maintain 

 themselves concentrically about the equator in a position that reminds 

 one of the rings of Saturn, the tension of the paper and its rapid vibra- 

 tion clearly showing that it is the repulsion of the equatorial current 

 that maintains them. 



