422 SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



that the rotatory motion contributes considerably to the increase of the 

 central depression, and that only through it does the duration of the 

 barometric minima, often for weeks, become possible ; and in my opinion 

 the fact that no cyclone has ever yet been observed in the neighbor- 

 hood of the equator finds an equally simple and sufficient explanation 

 in the absence of any motive toward a rotatory movement. 



Only in one not unimportant point do our views still differ. I think:'' 

 "The ascending current of air can only continue so long as its tempera- 

 ture, in consequence of the liberated latent heat of the vapor, exceeds 

 on the average that of the neighboring air-strata. But, on account of 

 this higher temperature, the pressure under the ascending current must 

 be less than that on the borders of the cyclone." On the other hand, 

 Hann says: "The condensation of the atmospheric aqueous vapor has 

 no sensible influence on the change of atmosj)heric pressure; the low 

 barometer in the storm-center can, therefore, not be explained by pre- 

 cipitation." Mohn's theory of the advance of cyclones toward their 

 rainiest side is therefore reliable or not, according as mine or Hann's 

 view is correct.^ 



Hann establishes his principle chiefly upon his reduction of 14G heavy 

 rainfalls in Batavia, that are recorded in the three-years' hourly obser- 

 vations of Dr. Bergsma. The barometer rose at the commencement, 

 and during the first two hours of the rainfall, on an average about 0.35 

 millimeter, but fell again about 0.1 millimeter within the two hours 

 following the rain, and therefore, after the rain, stood always 0.25 milli- 

 meter higher than before it. " This slight increase of pressure," Hann 

 very correctly thinks, "is a secondary eflect of the condensation of the 

 vapor; that is to say, it is produced by the cooling of the lower air- 

 strata by the raindrops that fall down from higher, colder strata, and 

 by the cooling due to evaporation. Perhaps, also, the concussion of the 

 falling water and of the air dragged along with it contributes somewhat 

 to the initial rapid rise of the pressure." 



To these purely local observations at Batavia we can now oppose a 

 longer series of observations, that embrace an extended country, and lead 

 to opposite conclusions, namely, the daily observations that were made 

 about 1S50 in the United States by numerous naval and Army officers, 

 and by 163 private individuals. James Espy for years entered these 

 upon synoptic charts. From more than 1,800 such charts he draws, prin- 

 cipally for the winter months, the definite conclusion :* "In all great and 

 sudden depressions of the barometer there is much rain and snow, and 

 in all sudden great rains or snows there is a great depression of the 

 barometer near the center of the storm and rise beyond its borders." 



Loomis, whom we also thank for other very important works on tor- 

 nadoes and storms, has lately discussed the weather-maps of the United 



-See this Journal, viii, p. 177 ; compare, also, my "Wirbelstlirme," p. 1'32. 



3 See, on the other hand, ix, pp. 344, 345, and x, p. 69. 



■• Espy, Fourth Melcorologkal Report. Washington, 1837. p. 10, Generalization No. 9. 



