SUMMER MEETING AT BROOKFIELD. 117 



is extremely weak, until by the meeting and convalescing of very small 

 drops, drops sufficiently large to fall to the earth as rain are formed. 

 By this theory, whenever an ascending current is induced from any 

 cause, if there is a considerable amount of vapor in the air, rain more or 

 less is produced, and the more rapid the ascending current and the 

 moister the air, the heavier is the rainfall. It is well known that large 

 fires, such as prairie fires or those of burning buildings, sometimes give 

 rise to a small shower of rain. This is by so heating and expanding the 

 air that it becomes much lighter than the surrounding air, and so rises 

 up and causes an ascending current, in which the vapor is condensed 

 into rain, as explained above. Espy suggested that, in times of great 

 drouth in the western drouthy States, an incipient ascending current thus 

 started artificially by means of prairie fires or the combustion of large 

 quantities of brushwood might be continued for some time and increase 

 and thus give rise to refreshing rains in time of drouth over a large 

 area of country. But this was regarded as being rather visionary and 

 entirely inadequate to produce much rain, and so the experiment was 

 never tried. 



Nature produces all our ordinary rain-storms by operating in a 

 similar manner, but upon a larger scale. AVhenever the air over a con- 

 siderable area of country becomes, for some reason, a little warmer 

 and damper than that of the surrounding parts, it rises up ; a warm, 

 •damp air near the earth's surface on all sides flows in to supply the 

 ascending current, and as the air ascends the moisture in it is con- 

 densed into cloud and rain. And the ascending current being once 

 started, the latent heat given out in condensation tends to further heat 

 the air in the ascending current and thus gives additional energy to it. 

 This is, in fact, generally so great that the ascending current must con- 

 tinue as long as the air which supplies it contains a considerable 

 amount of moisture, but after a while, when this is somewhat exhausted 

 -and the ascending current is supplied with air with little moisture in it, 

 the energy is diminished and the ascending current and the rain cease. 



If the air ascends over certain regions of the country, it must of 

 course descend over other, and whenever the latter occurs, which is 

 generally only a very gradual and imperceptible settling down of the 

 air, there is clearing or clear weather. For if there is cloud or fog in 

 the air when it first begins its downward motion, as it descends it be- 

 comes warmer and acquires greater capacity for moisture, and then the 

 • cloud particles evaporate and the air becomes clear. 



CYCLONE. 



In the flowing in of air from all sides in the lower part of the at- 

 mosphere toward the center of rarefaction, there is generally a gyration 



