SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 389 



times as great. Imagine a vertical column of one square meter section 

 isolated from the atmosphere, and assume tbat a kilogram of water 

 falls therein as rain, so that the rainfall is 1 millimeter. The air there- 

 upon expands considerably, and at a temperature of 10° C. at the place 

 of condensation 7.3 kilograms of air, on account of this expansion, either 

 are pressed to one side or are pressed upward, where they can flow to 

 one side. The weight of the column of air, which on the average 

 amounts to 10,33G kilograms, or one atmosphere, has therefore 

 diminished by 8.3 kilograms, including the condensed aqueous vapor. 

 The average barometric pressure must therefore have diminished by 

 8.3 -^ 10336 X 7G0 = 0,G1™'" : therefore f of the depth of rainfall. This 

 makes the fall in the barometer about 7 lines, or nearly 16 millimeters, 

 for an inch of rain. Of course this computation gives only the maxi- 

 mum change in the barometer, which in reality is by far not attained, 

 for the outpressed air cannot immediately flow away, and, moreover, is 

 at the earth's surface in great part replaced by colder air flowing from 

 either side ("Die Wirbelstiirme," page 215, and Pogg. Ann., 1865, vol. 

 cxxv). Kronig has in a similar way endeavored to prove, in opposition 

 to Mohr, that the expansion overbalances the contraction (Pogg. Ann., 

 1864, cxxiii). 



By both physicists the computation is conducted under conditions 

 that do not occur in nature. Vapor condenses, not spontaneously, but 

 only when the air is cooled; and this cooling must be greater than the 

 liberated latent heat, or else it certainly cannot cause any precipitation. 

 There remains, therefore, no surplus heat that can expand the air to 

 more than its previous volume. The latent heat of the vapor has simply 

 the effect of diminishing the cooling ; that is to say, it replaces a por- 

 tion of the heat that is lost. But a cooling must take place, and there- 

 fore, if we, in the above-imagined column of air, would bring a kilo- 

 gram of vapor to condensation, we must take away the whole of its 

 latent heat, and also deprive the air of so much heat as corresponds to 

 the contemplated reduction of the vapor-tension. The mass of air will 

 therefore contract ; consequently new air will flow in from above or from 

 the sides, and the barometer at the base of the column of air must rise. 

 This is the process which takes place in nature during the precipitations 

 that occur by reason of the inflow of a colder current of air, or by reason 

 of cooling in consequence of radiation and conduction. 



In the precipitations of ascending currents, heat is consumed in the 

 expansion of the air, and any accompanying condensation of vapor and 

 liberation of latent heat has the effect of diminishing the cooling of the 

 air. 



When we overlook this circumstance, we come, as Wottsteiu did, to 

 the question. Whence comes the heat rendered latent by the condensa- 

 tion of vapor? and to the opinion that a heavy precipitation is impos- 

 sible in the atmosphere if this heat is not immediately converted into 

 electricity'^ {Die Bezicliung tier ElcMricitdt zum Geiciffer, VierfeJjaJirs- 



