64 Professor CErsted on Water-Spouts. 



ascend, the only direction in which they can yield to the pres- 

 sure. We may be easily conA inced tliat this action also takes 

 place in the open an-, by distributing smoke in the air, from 

 a tobacco pipe for example, and then, at a distance of one 

 or two feet, producing a rapid rotatory movement, when the 

 extension of the whirlwind becomes appai-ent by means of the 

 smoke. 



Owing to the rotatory motion, the particles in the middle 

 must also have a centrifugal action, and there mu%t thus arise 

 a great rarefaction of the air at the centre. So long as the 

 whirlwind does not reach the earth, the air must ascend, to 

 fill up the vacant space which has been left by the particles 

 of air proceeding outwai'ds. The air must therefore stream 

 in anew from all directions, so that, when it has no particu- 

 larly great progressive rapidity, those objects which are car- 

 ried round by it must be dkected to a common centre ; but 

 when the progressive rapidity is great, the influence of both 

 forces on the direction must be perceptible. The rotatory 

 movement does not affect those crn'rents flowing inwards, in- 

 asmuch as it is taken for granted in this case, that the water- 

 spout, although very near the earth, has not touched it ; 

 for, in the latter case, the centrifugal force would also drive 

 outwards the particles of air near the earth. So long as the 

 water-spout does not reach the earth's sm-face, an ascending 

 current must prevail in its interior, which here constitutes the 

 elevating power. Wlien it strikes buildings, it may very often 

 happen that the inward flowing currents from below become 

 either entirely or almost altogether stopped. There thus 

 arises a great rai'efaction of the air around and over the build- 

 ing, so that the included air must drive the windows and walls 

 outwards, and must at the same time force upwards roofs, 

 and other objects which have air under them. 



A tube of the length formed by the centrifugal force of the 

 water-spout cannot be sufficiently filled from below by the 

 inward flowing crn'rents. A portion of the cloudy mass must 

 hence descend into the vortex. It is naturally the portions 

 nearest the middle which are driven with the greatest force 

 downwards ; nay, in a certain state of matters, the portions 

 in sinking will be altogether stopped by the centrifugal force. 



