. The Earth beneath the sea : History . (a) Fig. 2. (a) Continuous contact friction; (b) intermittent contact friction by diffusion of grain momentum. Taking an aloof view of the phenomenon as a whole, it is at once apparent that aU its aspects without exception share several common characteristics. (i) The sediment is heavier than the fluid. Its grains are, therefore, puUed towards the bed by a gravity component normal to the bed siu'face. If a mass m of sediment of density ps overlies unit area of any plane parallel to the bed surface which is inclined at an angle j8 to the horizontal, equih
. The Earth beneath the sea : History . (a) Fig. 2. (a) Continuous contact friction; (b) intermittent contact friction by diffusion of grain momentum. Taking an aloof view of the phenomenon as a whole, it is at once apparent that aU its aspects without exception share several common characteristics. (i) The sediment is heavier than the fluid. Its grains are, therefore, puUed towards the bed by a gravity component normal to the bed siu'face. If a mass m of sediment of density ps overlies unit area of any plane parallel to the bed surface which is inclined at an angle j8 to the horizontal, equihbrium demands a normal stress to be exerted across the plane, of value [{ps —p)lps]gm cos ^ whether the sediment is moving or stationary. Here p is the density of the pervading fluid. (ii) There must also exist in all cases a tangential stress [{ps -p)jps\gm sin /3 due to the tangential component of gravity. (iii) In all cases the movement of sediment over the bed involves the shearing of the pervading, inter-granular fluid and a shearing of the sediment grains over one another and over the bed surface. (iv) The shearing of any layer of grains settled on the bed and in static contact with one another requires, as O. Reynolds showed, some degree of dispersion or dilatation of the grains from one another. (v) In aU cases this dispersion must necessarily be upwards normally to the bed surface and therefore against the normal component of gravity.
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Keywords: ., bookcentury1900, bookcollectionbiodivers, booksubjectoceanbottom
