April 20, 1844. 



On the Transport of Erratic Blocks. By W. Hopkins', M.A., 

 F.R.S. &c. 



The principal object of this paper is to investigate the transport- 

 ing power of currents of water in general, and to explain in parti- 

 cular the nature of those which would arise from the instantaneous 

 or paroxysmal elevation of any considerable extent of the earth's sur- 

 face lying beneath the surface of the sea. The author has termed 

 them elevation currents. The immediate effect of an elevation like 

 that just supposed, would be the elevation to a nearly equal height, 

 of the surface of the superincumbent water, whence a great wave 

 would diverge in all directions. Such a wave would be attended by 

 a current in the direction of the wave's propagation, and has thence 

 been called a wave of translation. When such a wave proceeds along 

 a uniform canal, Mr. Russell has established experimentally the fol- 

 lowing facts : — 



1. Every particle in the same transverse section of the canal has 

 the same motion. 



2. The velocity with which the wave is propagated is equal to that 

 due to half the height of the crest of the wave above the bottom of 

 the canal. 



From these data the author has calculated the velocity of the cur- 

 rents which would necessarily attend these waves of elevation. It 

 depends principally on the height of the elevation and the depth of 

 the sea, while the time during which the current will flow depends 

 principally on the extent of the elevated area and the depth of the 

 sea. Thus if the depth of the sea should be 300 feet, and the height 

 of the crest of the wave above the even surface of the sea (which may 

 be considered as approximately the same as the elevation of the sud- 

 denly raised area) should be 50 feet, the wave would be propagated 

 with a velocity of upwards of 70 miles an hour, and the attendant 

 current would be upwards of 10 miles an hour. Also, if the elevated 

 area were circular, the width of the wave would exceed the radius 

 of the circle. The wave would have the essential character of a tidal 

 wave termed a bore, except that it diverges in all directions, instead 

 of proceeding along a confined channel. 



The author next proceeds to calculate the motive power of currents 

 of water. Let v be the velocity of the current, f , the density of the 

 water, and S the area of a plane surface on which the current acts, 

 and so placed as to make an angle 9 with the direction of the cur- 

 rent ; then if R denote the whole normal action of the current on S, 

 we have 



R = Jilo. Ssin 2 8, 

 2 ri 



provided 9 do not deviate too much from 90°. When 9 = 90° the 

 truth of this formula has been proved by numerous experiments, for 

 all velocities up to 11 or 12 miles an hour, and may be assumed to 



