428 SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



assume a peqietually renewed conjunction of winds in the necessary 

 direction continuously along the whole length of the storm-jiath through 

 the most diverse systems of currents. 



The new theory of the origin of the rotatory movement through the 

 deviating influence of the earth's rotation upon the air currents flowing 

 toward the area of least pressure fully explains the invariable direction 

 of rotation from right to left in the northern and from left to right in 

 the southern hemisphere. It also satisfies us esi)ecially in that it points 

 to the source whence the whirl when once formed steadily derives new 

 force. To this end it utilizes the fact that every great whirlwind is ac- 

 companied by an abundant condensation of aqueous vapor. The latent 

 heat liberated by this condensation causes a more rapid ascent of air 

 over the whirlwind and thus induces beneath it an inflow of the air from 

 all sides. AVe now see that the whirl can progress, indeed, that it must 

 progress if it is to have duration. The force that it needs to overcome 

 the frictional resistance, to draw in hitherto quiescent masses of air, to 

 bring about its powerful mechanical effects, — this force is already stored 

 up above in the atmosphere in the neighborhood of the path along which 

 it will pass. It remains latent until the approaching storm liberates it. 

 The whirlwind must quickly come to a stand when it does not find suf- 

 ficient aqueous vapor in the atmosphere and at the same time has great 

 frictional resistances to overcome. 



Since in this way this newer theory affords us a sufficient physical 

 reason for some of the most important phenomena observed in connec- 

 tion with storms, I have allowed myself to briefly call it the " physical 

 theory." 



We must, however, consider the explanation of the barometric minima 

 as due simjily to the condensation of aqueous vapor as not to an equal 

 extent satisfactory. I have already in this journal sufficiently fully 

 expressed my views as to the reasons that necessitate the rejection of the 

 view that by means of the condensation of aqueous vapor any consider- 

 able diminution of pressure at the earth's surface can be produced. 

 [See vols, viii, p. 102, ix, p. 2S9, and x, p. 11.] A principle correct in 

 itself has been pushed to extreme conclusions, to which the theory gives 

 no foundation and facts afford no agreement. Whoever desires to see 

 this one-sided application pushed to its extreme, may read the work of 

 Hopkins "On the Atmospheric Changes," and Laughton's "Physical 

 Geography." With many meteorologists at i)resent the aqueous vapor 

 is the great resort in every time of need, as was electricity, under the 

 influence of Volta's ideas. 



When one thinks himself obliged to reject a theory that appears to 

 explain satisfactorily one group of phenomena, he also to a certain ex- 

 tent assumes the duty of offering a better one. I have, however, just 

 as I was forced to think of attempting this duty, had the happiness 

 to find that this is no longer necessary; that in fact other and better 

 men have already done this. 



