9 o POPULAR SCIENCE MONTHLY. 



Dome extreme moisture may succeed almost absolute dryness in a 

 few instants, in a clear sky and without any change of wind. 



The parts of the atmosphere included within the same limits 

 of temperature or humidity therefore rarely form concentric or 

 parallel layers. They constitute regions interlacing zones which the 

 clouds, thick or scattered in groups, often mark to our eyes, show- 

 ing us those which are saturated with the vapor of water. The dis- 

 tribution of these zones in space depends chiefly on the heat action 

 of the sun, and upon preceding and present movements of the atmos- 

 phere. Now, if a certain increase of heat is adequate to melt a 

 piece of ice and to transform the water of the fusion into vapor, 

 in like manner a corresponding cooling may suffice to cause the 

 vapor to return to the state of a liquid and then to that of ice. The 

 processes in the atmosphere are not different, and all showers, results 

 of the more or less extensive condensation of the vapor of water, 

 may arise from the cooling of that vapor or of the water which 

 it produces. 



A certain volume of atmospheric air is capable of holding in 

 suspension a quantity of water proportioned to the elevation of its 

 temperature. But, for each determined temperature, there is a maxi- 

 mum which can not be exceeded without the excess of vapor return- 

 ing to the liquid state. If, therefore, an atmospheric region is 

 saturated with vapor, and its temperature falls, that region will give 

 rain. Immense and superabundant causes for the cooling necessary 

 to provoke rain exist in such an atmosphere as we have described. 

 The cooling may take effect in three principal ways: first, by the 

 radiation of different regions between one another and toward inter- 

 planetary space, the temperature of which is extremely low, as has 

 been indicated by measurements made in high balloon ascensions; 

 second, by the expansion which air rising in the atmosphere under- 

 goes in being rarefied ; and, third, by the mingling of masses of warm 

 or moist air with cold or dry. 



Cooling by mixture is the sufficient cause in the majority of 

 cases; and this may be effected from above, by descent of the air 

 from the upper regions; from below, by ascent, with the assistance 

 of rising currents created by solar radiations; or, finally, in any and 

 every direction under the influence of the winds and the general 

 movements of the atmosphere. Furthermore, the cooling need not 

 be very great in order to provoke rain under certain conditions of 

 temperature and humidity of frequent occurrence. 



Eain clouds very frequently descend a little below the altitude 

 of the Puy-de-D6me. It is, therefore, not difficult, in order to deter- 

 mine the degree of cooling necessary for the formation of rain, to 

 take advantage of observations that have been made there. The 



