APRIL. 163 



clouds, and thereby inducing that non-electric state wliich is 

 necessary to cause them to discharge rain. An organized 

 cloud is an aggregation of vaporous globules which are sus- 

 pended in the atmosphere, and held in this state of ujiion 

 without contact, by being in a similar electrified condition, 

 and kept separate according to that law of electricity which 

 causes two pith balls, similarly electrified, to repel one another 

 to a certain distance. All those clouds which have a definite 

 or organized arrangement of parts are charged with electricity; 

 there are others which are unorganized : such are the vapors 

 that rest on the surface of the earth, and which do not pro- 

 duce rain, except by acting as conductors between the earth 

 and the organized clouds in the upper atmosphere. 



To illustrate the influence of trees in producing showers, 

 we will suppose a dense cloud in an electric condition to be 

 passing over a desert plain : so long as it encountered no 

 conducting object in its journey, it would remain suspended 

 in the heavens until it met another cloud in an opposite 

 state of electricity, or until it was absorbed into the atmos- 

 phere. But if it should pass over a forest sufficiently elevated 

 to exert an influence upon it, the trees, which are good con-, 

 ductors, especially in summer when their sap is flowing, 

 Avould draw down the electricity from the cloud into the 

 earth. At that moment all the vaporous particles of the 

 cloud, no longer repelling one another, are mingled together 

 by the wind, and coalesce into drops of rain which descend 

 to the earth in showers. By this principle we may explain 

 the cause of a phenomenon which we often witness in a dry 

 season on the coast. A dense cloud is seen to pass over our 

 heads without aff"ording one drop of rain until it reaches the 

 ocean ; when the vapors that rest on the surface of the waves 

 acting as conductors, cause the cloud to part with its electric 

 fluid, and to fall In copious showers, at the same moment. 



I would not assert that the foliage of trees produces more 

 humidity by exhalation than the same amount of foliage of 

 herbaceous plants : but a square acre of ground covered with 

 trees is productive of a vastly greater quantity of foliage than 

 a square acre covered with any description of herbaceous 



