344 PRACTICAL LESSONS IN SCIENCE. 



that the power of the current is increased 64 times by doubling- 

 its velocity. A current of three inches per second will begin to 

 work on fine clay, one of six inches per second will lift fine sand, 

 and one of eight inches per second will move coarse sand, and one 

 of one foot per second will move fine gravel; while a current of 

 two feet per second, or a mile and a third per hour, will roll along 

 pebbles an inch in diameter, while a current of two miles per hour 

 will move stones the size of an egg. These figures are given for 

 the bottom velocity, and it should be remembered s'tones are not 

 more than two-thirds as heavy in the water as in the air, so that 

 they are more easily transported. 



In the upper course of the stream large angular blocks of rock, 

 loosened by the frost, are urged down the steep slope by the 

 rushing torrent ; these, gradually abrading each other and the 

 bed of the stream, in time reach the sea as gravel, sand or mud. 

 Some fragments may have reached the sea within a year after 

 leaving their mountain bed, while the last particles of some of the 

 blocks may be thousands of years in reaching their final destina- 

 tion in the bed of the ocean. 



The amount of solid matter conveyed by a large river to the 

 sea each year is almost beyond comprehension. Many investi- 

 gations have been made, but the most extensive and reliable are 

 those made by Humphrey and Abbott, on the Mississippi river. 

 They estimated that the amount pushed along the bottom of the 

 river into the Gulf, was 750,000,000 cubic feet annually, and the 

 amount in suspension was about 4,368,280,000 cubic feet, a to- 

 tal of 5,116,280,000 cubic feet per year, equal to a prism 268 

 feet high, with a base of one square mile. This means that the 

 Mississippi moves about one foot from the surface of its valley 

 in 6,000 years. Other rivers erode more rapidly, and some less 

 rapidly, but the above, as an average rate, would carry North 

 America into the sea within 4,000,000 years. 



Rivers erode their channels, but clear water erodes very slowly, 

 as shown in the case of the Niagara and St. Lawrence rivers, 

 which have had very little effect on the glaciated rocks in their 

 beds, during the 3,000 or 4,000 years since glacial times. The 



