THE MISSISSIPPI VALLEY. 3? 



The total drainage of the Ohio, above Wheehng, for the year 1845, was but — ■ 

 555,482,000,000 cubic feet ; 

 and the average daily discharge — 



1,522,000,000 cubic feet 



Comparing these vi^ith the corresponding results for 1847, we observe the 

 curious fact, that rather more than twice as much water was shed from the 

 country drained by the tributaries of the Ohio, in the year 1847, as in 1845. 



It is possible that an inspection of the observations on the fall of rain for 

 those two years, in the same region, may show that the actual fall was greater 

 in 1847 than in 1845 — a fact which would alone explain this result. But it is 

 by no means certain that the year in which there is the greatest number of 

 inches of rain and snow indicated by the gauge, is also that which will exhibit 

 the greatest discharge by the rivers of tlie country. The volume discharged 

 depends more on the season when the rain falls, and the quantities falling in a 

 given period, than upon the actual annual supply. Much of the winter^s rain 

 lies congealed upon the mountains, and at the northern sources of the stream, 

 and suffers but little loss from evaporation, until the first general thaw dissolves 

 the snows, and the whole quantity collected is poured off in a flood; while that 

 which falls in summer showers is evaporated at once, or absorbed by the dry 

 earth, to be slowly evaporated before it can affect the springs. 



A slight change of circumstances affects the annual discharge. The early 

 winter may be too mild to produce heavy snows, and the rain may pass off in 

 floods in December •, or it may be so severe as to freeze all the moisture that 

 reaches the earth, so that it will be retained in the hills until the approach of 

 the next spring. If the water which falls, runs off in December, it will affect 

 the discharge for the year which closes with December. But if retained in 

 the form of snow, it will be held over, and serve to swell the discharge of the 

 succeeding spring. 



These irregularities may be accounted for, to a great extent, without assuming 

 corresponding irregularities in the fall of rain. Yet, the difference between the 

 quantities which passed from the upper Ohio to the Ocean, in 1845 and 1847 

 respectively, may really be due, in part, to a difference in the amounts of rain 

 which actually fell in the i-espective years. The writer has neither library nor 

 meteorological record at his command, and cannot now properly investigate this 

 branch of his subject. 



The quantity of water which flowed down the Ohio in 1845, was sufficient to 

 have maintained an uniform depth on the bar at Wheeling, throughout the year, 

 of 7ilij feet. 



The drainage of the whole country, for this year, was equal to tVVu of a 

 foot ; or, 



QxViT inches. 

 Art. 4 — 6 



