s burden which is discharged into them by their
numerous tributaries, and by what means they are enabled to convey the
whole mass to the sea? If they had not this removing power, their
channels would be annually choked up, and the valleys of the lower
country, and plains at the base of mountain-chains, would be continually
strewed over with fragments of rock and sterile sand. But this evil is
prevented by a general law regulating the conduct of running
water,--that two equal streams do not, when united, occupy a bed of
double surface. Nay, the width of the principal river, after the
junction of a tributary, sometimes remains the same as before, or is
even lessened. The cause of this apparent paradox was long ago explained
by the Italian writers, who had studied the confluence of the Po and its
feeders in the plains of Lombardy.

The addition of a smaller river augments the velocity of the main
stream, often in the same proportion as it does the quantity of water.
Thus the Venetian branch of the Po swallowed up the Ferranese branch and
that of Panaro without any enlargement of its own dimensions. The cause
of the greater velocity is, first, that after the union of two rivers
the water, in place of the friction of four shores, has only that of two
to surmount; 2dly, because the main body of the stream being farther
distant from the banks, flows on with less interruption; and lastly,
because a greater quantity of water moving more swiftly, digs deeper
into the river's bed. By this beautiful adjustment, the water which
drains the interior country is made continually to occupy less room as
it approaches the sea; and thus the most valuable part of our
continents, the rich deltas and great alluvial plains, are prevented
from being constantly under water.

_River floods in Scotland_, 1829.--Many remarkable illustrations of the
power of running water in moving stones and heavy materials were
afforded by the storm and floods which occurred on the 3d and 4th of
August, 1829, in Aberdeenshire and other counties in Scotland. The
elements during this storm assumed all the characters which mark the
tropical hurricanes; the wind blowing in sudden gusts and whirlwinds,
the lightning and thunder being such as is rarely witnessed in our
climate, and heavy rain falling without intermission. The floods
extended almost simultaneously, and with equal violence over that part
of the northeast of Scotland which would be cut off by two lines dra