Cii. XV. I 



TRANSPORTING TOWER OF WATER. 



347 



point from which it started. The extent of these curves 

 depends on many conditions, especially on the nature and 

 tenacity of the alluvial soil often strengthened by the roots of 

 buried* trees, and on the slope or fall of the river's bed. 



When the tortuous flexures of a river are extremely great, 

 the aberration from the direct line of descent may be restored 

 by the river cutting through the isthmus which separates 

 two neighbouring curves. Thus, in the annexed diagram, the 

 extreme sinuosity of the river has caused it to return for a 



Eig. 20. 



ary 



a peninsula is formed, and the isthmus (at a) is consumed on 

 both sides by currents flowing in opposite directions. In this 



case an island is 



formed, — on either side of which a 



portion of the stream usually remains. 



Transporting power of tvater. — In regard to the transporting 

 power of water, we may often be surprised at the facility 

 with which streams of a small size, and descending a slight 

 declivity, bear along coarse sand and gravel ; for we usually 

 estimate the weight of rocks in air, and do not reflect on 

 their comparative buoyancy when submerged in a denser 

 fluid. The specific gravity of many rocks is not more than 

 twice that of water, and very rarely more than thrice, so 

 that almost all the fragments propelled by a stream have lost 

 a third, and many of them half, of what we usually term 

 their weight. 



It has been proved by experiments, in contradiction to the 

 theories of the earlier writers on hydrostatics, to be a 

 universal law, regulating the motion of running water, that 

 the velocity at the bottom of the stream is everywhere less 

 than in any part above it, and is greatest at the surface. 

 Also, that the superficial particles in the middle of the 

 stream move swifter than those at the sides. This retarda- 

 tion of the lowest and lateral currents is produced by 

 friction ; and when the velocity is sufficiently great, the soil 



