IX.] THE WORK OF RAIN AND RIVERS. 133 



the gravel is the first to fall ; then the sand subsides, and 

 finally the mud settles down. 



If a river has a steep bed it generally possesses great 

 carrying power. Mountain-torrents, for example, rush down 

 steep slopes and not only transport vast quantities of gravel, 

 sand and mud, but often move stones of considerable 

 weight. During floods, too, ordinary rivers acquire great 

 mechanical power. Thus we read of floods sweeping away 

 bridges, tearing up rocks from the banks of the river, and 

 carrying along stones weighing several tons. Sir T. D. 

 Lauder, in describing the great floods which occurred in 

 Morayshire in August 1829, records the destruction of many 

 farms and hamlets ; while no fewer than 38 bridges were 

 swept away by the flooded rivers. A huge mass of sand- 

 stone, measuring 14 feet in length, 3 feet in width, and one 

 foot in thickness, was carried for a distance of 200 yards by 

 the swollen stream of the river Nairn. 



In estimating the carrying-power of running water, it must 

 be borne in mind that the weight of a stone is considerably 

 less in water than in the atmosphere. When a body is im- 

 mersed in water, it appears to lose a certain proportion of 

 Its weight, the proportion depending upon its specific 

 gravity. If a stone be twice as heavy as an equal bulk 

 of water, it will lose one-half its weight ; if three times as 

 heavy, it is lightened by one-third ; and so on. It is 

 generally said that if a stream flow at the rate of six inches 

 per second it has power enough to carry off fine sand ; if 

 at 12 inches per second it can sweep away fine gravel .; and 

 if at 36 inches per second it can transport pebbles as large 

 as hen's eggs. It should not be forgotten, however, that the 

 shape of the fragments greatly aff"ects the ease with which 

 they may be moved in water. 



Hitherto, the work of the river has been regarded as 



