Geology. 123 
surface, according to the author, is two per cent. less than the velocity 
below the surface to near the bottom, and he attributes the surface re- 
tardation to contact with the atmosphere. The amount of water dis- 
charged by the Mississippi (including the discharge of the Atchafalaya) 
at the flood of 1850 is stated at 1,280,000 cubic feet per second. The 
discharge below New Orleans, at the top of the flood of 1851, was 
995,000 cubic feet per second; and at Memphis, in 1850, 958,500 cubic 
feet. It isa remarkable fact, as the author observes, that at the same time, 
the discharge of water higher up the stream was much greater than this. 
For example, one mile below the mouth of the Ohio in June, 1851, it was 
1,223,000 cubic feet, although the flood was 7 feet 10 inches below the 
high water of 1850; and in July 1851, at Cape Girardeau, above the 
mouth of the Ohio, when the water was 4 feet below the high water of 
1850, it was 1,025,000 cubic feet per second. These quantities are the 
e 
swamps of the southern counties of Missouri and the northern counties 
of Arkansas and escapes the measurements at is. 
The author treats of the local changes and irregularities of the river, 
arising from the crevasses, the building of levees, a change in the 
bends, the action of the winds, and variations in the tributaries. In the 
course of his remarks on the influence of the winds, he mentions that 
harrow sheet of water, he detected a variation in the surface of more 
than 8 inches in 20 miles, produced by a continued but moderate breeze. 
For the important discussions of the author upon the different modes 
of protection against floods, we must refer to his work. 
e close by the following citations upon the Ohio river. 
“This noble tributary rises on the borders of Lake Erie, at an aver- 
age elevation of 1,300 feet above the surface of the sea, and nearly 
flows is connected with no mountain range at its northern extremity, 
but continues its rise with great uniformity, from the mouth of the 
Ohio to the brim of the basin which incloses Lake Erie. The sources 
of the tributary streams are generally diminutive ponds, distributed 
along the edge of the basin of Lake Erie, but far above its surface, 
and so slightly separated from it, that they may all be drained with 
little labor down the steep slopes into that inland sea. 
hese remote sources, a boat may start with sufficient water, 
within seven miles of Lake Erie, in sight, sometimes, of the sails 
tle—so little accelerated by rapids—that when there is sufficient water 
to float the vessel, and sufficient power to govern it, the downward 
voyage may be performed without difficulty or danger in the channels 
as they were formed by nature; and the return trip might be made 
with equal security and success with very little aid from art.” —p- 231. 
