70 



SCIENCE 



[N. S. Vol. XXXVIII. No. I 



In the interior enclosed basins the pre- 

 cipitation and evaporation, as a rule, are 

 equal to each other. 



Bruckner's figures for entire earth's sur- 

 face are corroborated also by studies of spe- 

 cific drainage areas. The most interesting 

 study in this connection is that by Pro- 

 fessors Francis E. Nipher^ and George A. 

 Lindsay on the rainfall of the state of 

 Missouri and the discharge of the Missis- 

 sippi River at St. Louis and Carrollton, 

 Louisiana. Nipher found that the average 

 discharge of the Mississippi River at St. 

 Louis during the ten years ending Decem- 

 ber 31, 1887, was 190,800 cubic feet per 

 second. The amount of water falling per 

 second upon the whole state during the 

 same interval was 195,800 cubic feet per 

 second, or equal within two per cent, to the 

 discharge of the Mississippi River at St. 

 Louis. If, however, a comparison is made 

 between the total rainfall on the basin 

 draining past St. Louis and the river dis- 

 charge at this point, it appears that the 

 drainage area of the Mississippi and Mis- 

 souri Rivers above St. Louis is 733,120 

 square miles, or over 10 times the area of 

 Missouri. These figures show what small 

 portion of the total rainfall over the drain- 

 age basin of the Mississippi River is led 

 into the rivers and conducted back to the 

 sea. It is evident that by far the larger 

 portion of the precipitation that falls over 

 the drainage basin is evaporated back from 

 the land into the atmosphere, and is not 

 returned to the sea through the medium 

 of drainage. These figures show further 

 that the source of precipitation of the Mis- 

 sissippi drainage is from evaporation over 

 the land and not derived from evaporation 



'Francis E. Nipher, "Eeport on Missouri Eain- 

 fall, witli Averages for Ten Years ending Decem- 

 ber, 1887," Transactions of the Academy of Sci- 

 ence of St. Louis, Vol. v., p. 383. 



over the sea. Mr. Lindsay* computed the 

 discharge of the Mississippi River at Car- 

 rollton, Louisiana, and found that the av- 

 erage for fourteen years was 117 cubic 

 miles per year, or 545,800 cubic feet per 

 second, which is less than three times the 

 precipitation over the state of Missouri. 



The central portion of the United States 

 is distinctly a continental region, particu- 

 larly the prairie region, which suffers from 

 lack of precipitation. On the other hand, 

 large areas in the south and southeast suf- 

 fer from too much humidity because of 

 large swamps, which is caused not only by 

 excessive precipitation, but also by deficient 

 evaporation. Not only the south and 

 southeastern areas suffer from too much 

 water, but also many portions in the north 

 and northeast, where the evaporation is 

 also very slight. We have, therefore, two 

 extremes on the periphery of the United 

 States: (1) In the states adjoining the 

 Atlantic Ocean and the Gulf of Mexico 

 there is an excess of moisture on the 

 ground, both on account of excessive pre- 

 cipitation and slight evaporation; (2) in 

 the vast interior of the central United 

 States, on the other hand, there is a defi- 

 ciency of moisture, both on account of the 

 scant precipitation and of the intense 

 evaporation. Is there not some connection 

 between these two extremes? Is it not 

 possible that changes which take place in 

 one part of this vast region may exert 

 some influence on the condition of the 

 other? We have seen that in the central 

 states in summer the prevailing westerly 

 and northwesterly winds give way to 

 southerly and southeasterly winds. In 

 other words, in the summer the central 

 states are under the influence of moist 



* Geo. A. Lindsay, ' ' The Annual Rainfall and 

 Temperature of the United States," Transactions 

 of the Academy of Science of St. Louis, June, 

 1912. 



