CAUSES OF RAIN. 43 



or trough of the wave with a fall in the barometer. This 

 indicates the approach of wind. The high pressure under the 

 wave forces the air to the center of the depression and the 

 winds blow from all quarters to restore the pressure. Thus 

 with every wave which ruffles the surface of the vast ocean 

 above us, currents are set in motion around us, and the air 

 is never still. 



The watery vapor suspended in the air is affected by this 

 oscillation of pressure to some extent ; but far more by the 

 changes of temperature. When the air is heated, evaporation 

 from the ocean and the land is active, and the atmosphere 

 is charged with moisture. A wave rising in the north with 

 a depression in the south, brings a cold wind. This strikes 

 the warm moisture laden air, and the vapor is condensed 

 in clouds. The condensation increases with the fall of tem- 

 perature ; the clouds become heavy and black, and soon the 

 pattering rain drops fall upon the thirsty soil, refresh the 

 crops and gladden the husbandman. Or the friction of the 

 cool current passing over a warmer one, engenders elec- 

 trical disturbance, when the lightnings flash and suddenly 

 condense the gathered vapor, and the thunder showers pour 

 down amid the deafening crashes and reverberations. 



All these facts of surpassing interest and which explain 

 so simply the causes of these phenomena, are based upon a 

 series of natural laws, which are as beautiful as they are 

 wonderful, and they have been unfolded by science within 

 a few years past. Our fathers knew nothing of them, and 

 vainly imagined causes for them. But we knowing them, 

 find ever new delights in their contemplation. We see 

 that the vegetable world is derived in greater part from the 

 air, and consists of condensed gases that have been reduced 

 from the atmosphere by the agency of the sun's heat. 

 Animals which derive all the materials of their structure 

 from plants, destroy these while living by respiration, and 

 when dead by decomposition, and return them in gaseous 

 form to the air again, whence they were taken. Thus the 

 offices of plants and animals neutralize each other ; the one 

 takes the materials for its substance from the air and builds 



