Page:Popular Science Monthly Volume 2.djvu/685

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power of its velocity—that is, doubling the rate, increases the force sixty-four times. "If a stream running 10 miles an hour would just move a block of five tons weight, one of 20 miles would move one of 320 tons." When shallow streams are suddenly swollen to torrents, bowlders of considerable size are borne along with heavy roar—are broken by collision, and ground to pebbles and sand. Hence we find that heavy rock-masses are abundant near the sources of a river, finer materials along its valley, fine sand and silt at its mouth. The material thus comminuted is prepared for distribution over the terrace-flats and plains along the river's bed.

The following table is from Mather's "Report on the Geology of the State of New York:"

A stream having a velocity of—

3  inches per second wears away fine tough clay
6 "" removes fine sand.
12 "" removes gravel.
24 "" removes pebbles one inch in diameter.
36 "" moves angular fragments having a diameter of two or three inches.

The latter velocity is, however, very moderate, being little over two miles an hour. In estimating the transporting power of rivers, we must consider the important fact that rocks lose nearly one-third of their weight in water.

The formation of terraces on the borders of lakes has, equally with those along the banks of rivers, arisen from elevation of the land. By this means the drainage of Lake Superior became possible, and its terraces represent former levels of its waters. Nor less interesting is the fact that the great plains of Northern Africa, including the Sahara and the valley of Egypt, were emerging from the sea while the Nile was excavating its valley and carving into terraces the sands of the Nubian Desert more than 200 feet above the present bed of the river. The wave-eaten shores of Patagonia have been elevated above the ocean, and its terrace-plains, equally with those of lake and river valleys, constitute an epoch of geological history, and record the most profound of the earth's secular changes.


By J. R. BLACK, M. D.

IN the intellectual field there are two distinct classes of laborers: the discoverers, or those who pursue science for its own sake; and the appliers, or those who seek to make the knowledge it confers useful, and so turn it into a direct source of profit to themselves, and, in a general way, to the public. The genius each displays differs from the