250 



There can be little doubt that the great agents in moulding the surface of 

 the earth to its present shape, are, the cuiTents of water which are eternally 

 percolating through the soil, detaching particles of solid matter and conveying 

 them to a lower level. If we could but measure and weigh these solid consti- 

 tuents of our rivers we should have an estimate of the rate at which the rocks 

 are being denuded. Allow me for a moment to review the process by which 

 this denudation takes place. When we stand on some rocky mountain and 

 look into the gorges below, the sides of them are seen to be covered with 

 rough blocks hurled time after time from their place in the mountain side. 

 This has been effected chiefly by the action of frost splitting open the stones 

 and separating them into ever diminishing fragments shading off into the soil 

 of the lower parts of the valley. Again, if you examine the section presented 

 by any quarry, you see that the solid and thickly bedded rock beneath as it 

 is traced upwards to a level with the ground is more and more subject to 

 fissures until within perhaps a foot or two of the surface it becomes the fine 

 clay or loom which rejoices the farmer's heart. Every shower of rain then 

 which falls on this finely ground up rook or soil carries off its share to the 

 river below, and then it commences its long journey to the sea, the finest 

 portions being carried rapidly along in suspension, the heaviest awaiting the 

 impulse of some violent flood, and the intermediate pebbles and sand making 

 an occasional journey, sometimes halting long in some retired nooks where 

 they are protected from the force of the current, and anon distui-bed and 

 carried a considerable distance ; when, as sometimes happens, the local bars 

 and gravel heaps formed in comparatively quiet times are broken up. 



It is with the first of these portions, or the fine impalpable mud which 

 is carried down in suspension, that the experiments I am going to describe 

 have to do. The problem as regards sand, pebbles, and larger stones, pushed 

 and carried down the bed of the river, though not perhaps an insoluble one is 

 certainly much more difficult. 



My object then is simply to determine what quantity of solid matter 

 passes down a given spot, on the banks of a river, within a certain time. It is 

 clear that two elements are requisite to estimate this — 1st, the proportion of 

 sediment held in suspension in a certain measure of water, and secondly the 

 quantity of water that passes during the flood. The first may be most simply 

 done by collecting at intervals during the flood an ordinary wine bottle full of 

 the muddy water. This holds, on an average, 26oz. of water. If then this 

 sediment is allowed completely to settle {a process which takes some ten to 

 14 days in the case of the river on which I have been experimenting) and then 

 the clear water poured off and the dregs filtered through a paper, which is care- 

 fully first dried and then weighed before and after the experiment, we have the 

 exact weight of mud in 20oz. of water. 



The detei'mination of the other element in our calculation, namely, the 

 volume of water which jiasses down a river in a given time, is a more diflScult 

 jiroblem ; indeed, it is erident that a great variety of things must modify 



