SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 415 



Tvben be says that in the whirlwinds the moving force is the latent beat 

 of the condensed aqueous vapor.* 



The results of the investigations of Ley and Blanford into the origin 

 of whirlwinds agree, as I believe, entirely with the preceding presenta- 

 tion of the case, that the barometric minimum at the center of a cyclone 

 is primarily a consequence of the whirling movement of the air; for it 

 does not happen that in consequence of extended heavy precipitations 

 a decided barometric depression forms over a region about which then 

 the air begins to circulate, but it is only after the whirl is well devel- 

 oped that an area of low pressure forms, which then progresses further. 

 From the weather prevailing over the Bay of Bengal previous to the 

 Calcutta cyclone of 18G7, Blanford has drawn the conclusion that the 

 cyclones originate in a region of calms and variable winds between the 

 northeast monsoon and the retreating southwest monsoon. Their pri- 

 mary cause consists in local accumulations and condensations of aqueous 

 vapor in a calm region above the ocean. Many days elapse before the 

 whirlwind has developed itself (Proceedings of the Royal Society, 18G9). 

 Similar are the conclusions of Ley as to the origin of whirlwinds in 

 England.^^ 



There still remains unexplained, so far as I can see, the cause which 

 determines the different intensities of the barometric minima in the 

 centers of storms in the tropics and in our latitudes in the summer and 

 winter. If the intensity of the precipitation determined this, then would 

 the barometric minima of the summer exceed those of the winter,^^ but it 

 seems more promising to seek for the cause of the greater intensity of the 

 barometric depressions in winter in the greater differences of temperature. 

 The air over the warm Atlantic Ocean, strongly warmed by frequent 

 precipitations, is then bounded on both sides by very cold dry air. This 

 must give rise to decided disturbances of equilibrium and to storms; 

 but, on the other hand, in the tropical cyclones the temperature differ- 

 ences can have no relation to the intensity of the storm. 



That in the condensation of aqueous vapor in winter the latent 

 heat of fluidity is, in the case of the freezing of water, to be added to the 

 latent heat of vaporization, can onlj'^ make a slight difference, because 

 the former amounts to only a little over one-eighth of the latter (see 

 page 29). The ratio of the liberated heat at 15° C. to that at— fP 

 0. is as 596 to 690 or as 1 is to 1.16. Herein, therefore, the expla- 

 nation cannot lie, as Espy, and in part also Reye, assumes. Rather 

 could we think that the circumstance that in winter the condensation 

 takes place in lower, denser strata of air, and at lower temperatures, 

 should increase the upward tendency. In fact, this must be the case, 

 but, as a simple computation shows, the buoyancy of a mass of air is, 

 for the same temperature differences, only about one-fifth smaller when 

 the pressure is 600 millimeters instead of 700 millimeters, and only 



[* Note by the Translator.— This theorem will also be fouud accepted by Feirel : 

 Mathematical MontlJy, ii, 1858.] 



