206 W J MC GEE OUTLINES OF HYDROLOGY 



repel and ground or settle extraneous matter up to a certain point, then 

 after adventitious loading to first gather and later triturate the foreign 

 particles until small enough for grounding, and eventually to clarify 

 itself even more effectively moving than standing, the general tendency 

 l)eing toward the elimination of extraneous or adventitious matter in 

 such manner as to reduce internal friction and consequent expenditure 

 of energy on its way from source to sea — a habit of prime practical im- 

 portance already discussed by Powell, inferred by Gilbert, and observed 

 by Leighton. Perhaps the chief merit of the concept lies in its relevancy 

 to the classic investigation by Osborne Eeynolds into "the circumstances 

 which determine whether the motion of water shall be direct or sinuous,'" 

 and its sufficiency as an interpretation of his experimental results re- 

 corded by the Eoyal Society a quarter-century ago ; for it is manifest that 

 under the condition of little external constraint (such as pressure or 

 declivity) a relatively small number of mutually frictionless particles 

 may adjust their movements to those of a relatively large number of 

 friction-restrained particles, while with either (a) increased constraint ov 

 (b) reduced ratio of friction-restrained particles the normal sinuous (or 

 saltatory) movement must recur — a habit also of practical consequence 

 as the basis of the law governing the flow of water in pipes. 



On the whole, the concept of streams as made up of discrete particles 

 moving saltatorily along paraboloid paths simplifies and coordinates views 

 of stream-work under the variable conditions of the earth-face; at the 

 same time it helps to harmonize the physics of the particular movement 

 with both terrestrial and celestial physics in their general aspects. 



CONCEPT OF THE WATER MODULE 



When viewed narrowly, in the light of various analogies, the general 

 behavior of running water suggests the nature of the water particle: 

 Clearly the discrete particle involved in stream movement can not be a 

 single molecule but must comprise a number of molecules of HjO, since 

 it must be of sufficient magnitude to undergo the change of state from 

 liquid to solid or gas, or vice versa, without disturbing the atomic bond ; 

 clearl)^, too, the particle can not be an air-formed drop constrained by 

 surface tension, since it is not isolated and also since surface tension 

 itself is due to interrelations of particles (sometimes unnecessarily con- 

 ceived as molecules) ; so that, proceeding by exclusion, a discrete aggre- 

 gation comprising several molecules yet smaller (probably much smaller) 

 than an air-formed drop may be inferred prima facie — an aggregation 

 perhaps of sufficient size to form an invisibly small particle of vapor or a 

 minute spicule of ice with the modification of movement among the mole- 

 cules themselves induced by change of temperature. 



