136 AGENTS THAT DESTROY ROCKS. 
greater quantity of soil than it would do if the rain 
had been distributed over the plains and mountains 
equally, in proportion to their relative areas. The 
rocks, also, in the higher regions are more exposed 
to atmospheric influences, such as frost, rain, and 
vapour, and to great alternations* of heat and cold. 
Besides, when earthy matter has once been inter- 
mixed with running water, anew mechanical power 
is obtained by the attrition of sand and pebbles, 
borne along with violence by a storm. Running wa- 
ter, charged with foreign ingredients, being thrown 
against a rock, excavates it by mechanical force, sap- 
ping and undermining till the superincumbent por- 
tion is at length precipitated into the stream. 
In estimating the transporting power of water, it 
is necessary to bear in mind that the specific grav- 
ity of many rocks is not more than twice that of 
water, and rarely more than thrice, so that almost 
all the fragments propelled by a stream have lost 
from a third to one half of their weight. This may 
be easily shown by lifting a stone in air, and then in 
WB-ter. It is also well to know that the velocity at 
the bottom of a running stream of water is every- 
where less than in any part above it, and greatest 
of all at the surface. Accordingly, the superficial 
particles in the middle of a stream move swifter 
than those at the sides. This retardation of the 
lowest and lateral currents is produced by friction ; 
when the velocity is sufficiently great, the soil com- 
posing the sides and bottoms gives \^y. A veloci- 
ty of three inches per second at the bottom is as- 
certained to be sufficient to bear up fine clay; six 
inches per second, fine sand ; twelve inchesf per 
second, fine gravel ; and three feet per second, stones 
of the size of an egg. 
A few examples will show the immense power 
* LyelPs Geology. 
t Encyc. Brit., Art. Rivers. 
