Observations of the United States 



r -four hours, either a; 



thus frequently happens that the atmosphere 'is in a state of 

 unstable equilibrium, and a slight cause is sufficient , to deter- 

 mine a strong upward movement of the surface air, resulting in 

 a precipitation of vapor. When air in this unstable condition 

 has a movement along the earth's surface, a slight inequality of 

 the surface may be sufficient to determine an upward move- 

 ment at that point. We should therefore conclude that when 

 a current of warm and moist air flows in from the ocean and 

 impinges upon the land, a strong upward movement accom- 

 panied by heavy precipitation must frequently result This 

 conclusion is confirmed by a comparison of the preceding obser- 

 vations. Among the stations of the Signal Service south of 

 latitude 36°, there are twelve stations situated on the Atlantic 

 coast or the Gulf of Mexico. At these stations during a period 

 of fifteen months the average number of great rain-falls (two 

 inches in eight hours) has been 3'6. Among the stations of the 

 signal service south of latitude 36°, there are also five inland 

 stations, whose average distance from the coast is 200 miles, 

 ami during the same period the average number of great rain- 

 falls at these stations has been 1-8 ; that is, near the coast great 

 rain-falls occur twice as frequently as they do at stations in the 

 interior at a distance of 200 miles from the coast. 



Whenever a strong upward movement of the air commences, 

 there must be a general tendency of the surrounding air toward 

 this point, and the air must circulate from right to left as it 

 does in great storms. Hence every great rain-storm should be 

 accompanied by an inward and cycloidal motion of the air. In 

 about two-thirds of the preceding cases there is decided evidence 

 of the existence of such a cycloidal movement, and in most of 

 the remaining cases there are some indications of such a move- 

 i could be obtained from a sufficient 



lumber of stations, it is believed that some degree of c 

 notion would be indicated in everv case of heavv rain-fall. 



The depression of the barometer attending these rain fa"s 

 vas generally small, nevertheless in several cases it was quite 



