648 VICTOR ZIEGLER 



of the parallelogram of forces constructed with velocity and gravity 

 as sides (Fig. 2). The grain will experi- 

 ence no resistance in the direction of the 

 velocity, as it will simply move along 

 with the water. The downward move- 

 ment will experience the same resistance 

 Fig. 2 as though the liquid were at rest. 



MOLECULAR FORCES AND TRANSPORTATION 



Sediment is transported by water in one of three methods. 

 It is either floated on the surface, or rolled along the bottom, or 

 carried in suspension. 



Small grains, when carefully sifted over the surface of water, 

 float, due to the fact that their weight is not sufficient to overcome 

 the surface tension of water. Since surface tension may be defined 

 as the "force tending to make a liquid contract to the smallest 

 area admissible," it will have the tendency to drive the floating 

 grains together. 1 This apparent attraction of grains into patches, 

 although not explained, has been noted by James C. Graham and 

 F. W. Simonds, who described this method of sand-transportation 

 as occurring on the Connecticut and Llanos rivers respectively. 2 

 Experiments carried on by Simonds seem to show that if the 

 launching be favorable, about 40 per cent of the component grains 

 of most sandstones will float on water. Floating patches of sand 

 and dust have been noticed by the author on the Iowa and Cedar 

 rivers on still days during the summer, where they look essentially 

 like floating patches of scum or foam, and also on the quiet 

 water along the shore of the North Sea, near Otterndorf and 

 Cuxhaven in Germany. While the condition necessary for the 

 transportation by flotation are somewhat unusual, this method still 

 appears to have more importance than is usually attributed to it. 



The floating of the grain depends on two molecular forces, 

 viz., cohesion and adhesion. Cohesion causes the tension in the 

 free surface of the water, and resists all attempts to break this 

 surface. Adhesion serves as a modifying factor. If the adhesion 



1 Duff, op. cit., 146. 



2 Graham, A.J.S., series 3, XL, 476; Simonds, Am. Geol., XVII, 29. 



