226 Mr. HOPKINS, ON THE TRANSPORT OF ERRATIC BLOCKS. 



11. Hence if a body once begin to roll, and we would calculate the force of the current just 

 sufficient to keep it in motion, we may consider the fluid pressures as acting on every part of its 

 surface, and our results will be very approximately true, independently of the nature of the surface 

 over which the motion takes place, provided that surface be sufficiently firm to give the requisite 

 support to the rolling body. The force, however, thus determined might be insufficient to make 

 the body begin to move, since it might rest in such a position as to exclude the fluid action from its 

 lower surface. But here it should be carefully observed, that a current is net to be deemed ineffi- 

 cient in moving blocks of given weight and form, unless it be capable of moving all such blocks ; 

 on the contrary, it is to be considered efficient for that purpose, if it be sufficient to move such of 

 them as may exist under conditions most favorable for transport. In many cases the incipient 

 motion might be due to accidental causes, as, for example, an impulsive blow from another mass 

 already in motion ; and, moreover, it is probable that all blocks which may have been transported 

 by this agency to considerable distances, have been carried on by currents of considerably greater 

 force than that just sufficient to keep them in motion, and which may liave been sufficient without 

 accidental causes to move them from rest, even under conditions not the most favorable for their 

 movement. 



The preceding remarks are of the first importance as removing all doubt and uncertainty with 

 respect to the applicability of our calculated results to actual cases of transport b)' the agency of 

 currents, whenever those results involve the hypothesis of the rolling motion of the transported 

 mass. Transported bodies may have moved by rolling or by sliding; but in the latter case, the 

 retarding action of friction and local obstacles introduces so uncertain an element as to render 

 calculation comparatively useless ; but if in calculating the force necessary to move a block of 

 considerable magnitude, we assume it to have moved by rolling, we avoid in a great degree the 

 uncertainty arising from the above causes, and are in no danger of assigning its transport to a force 

 much less than that which has been actually required for that purpose. 



We may now proceed to investigate the force which a current is capable of exerting on bodies 

 of particular forms. It will be sufficient for our purpose to take a few prismatic bodies, of which 

 the sections perpendicular to their axes are triangles, rectangular parallelograms, pentagons or 

 hexagons. Tliese cases will shew how the transporting power of a current, as estimated by the 

 mass it is capable of moving, depends on the form of the mass ; and will enable us to estimate, to a 

 sufficient degree of approximation, the velocity of a current capable of moving any proposed erratic 

 block. 



12. If a plain surface, whose area = .S" be placed at rest in a fluid, whose density is jOj, moving 

 with a velocity v, in a direction making an angle 6 with the plane, we shall have 



R = (p(e).- p,Sim'-6, 



R being the moving force of the current on the plane estimated in the direction perpendicular to 

 the plane ; and if R' be the resolved part of this force in the direction of the current, 



R =<p{0).~p,Ssm^9, 



which will be the whole force in this direction, if we neglect the friction between the fluid and the 

 plane. 



7]- 



When = —, numerous experin)ents, made by different persons, shew that 



R =-p,.S 

 very ai)|ir<)ximately. The experiments have been made with different velocities up to 11 or 12 



