Mr. HOPKINS, ON THE TRANSPORT OF ERRATIC BLOCKS. 225 



of the attending current may become much greater than in the uncontracted wave. Such must 

 have been the case with respect to the portion of a wave diverging from the district of the 

 Cumbrian mountains, and received into the strait which must have been formed by the 

 pass of Stainmoor previously to its emergence from the ocean, but subsequently to that of the 

 higher mountains to the north and south of it. 



We may now proceed to investigate the transporting power of currents originating in the 

 manner above explained. 



SECTION II. 

 Transporting Potver of Currents. 



9. Whatever be the specific gravity of a body, if its dimensions be sufficiently small, it 

 can never acquire more than a small velocity in descending by gravity in any fluid of which 

 the density is not extremely small. Such a body may therefore be held in suspension in water 

 for a considerable time, and when placed in running water, soon acquires a horizontal velocity 

 indefinitely nearly equal to that of the current. It may therefore be transported to considerable 

 distances before it descend to the bottom ; or when once deposited on the bed of the stream, it may 

 easily be again disturbed, and carried onward as before. When the body is not however of very 

 small dimensions it can only be transported along the bottom by the impelling force of the current, 

 its motion being retarded by friction, or the resistance of solid obstacles. In this latter case it is 

 necessary to ascertain the relation between the velocity of the current and the dimensions 

 and weight of the largest mass it is capable of moving. This relation depends not only on the 

 volume and specific gravity of the mass, but also on its form ; and therefore, in order to ascertain 

 whether certain given bodies could be moved by a given current, a separate investigation would, 

 in strictness, be necessary for each, supposing their forms to be different, though they might in all 

 other respects be the same. To render our results immediately applicable however, with sufficient 

 accuracy for our general purpose, it will be sufficient to investigate the above-mentioned relation 

 for a few certain forms, and then to refer any proposed mass to that particular form to which it 

 most nearly approximates, among those for which the above investigation has been made. 



10. A body acted on by a current may be moved by sliding or by rolling. In the former 

 case, a very uncertain element, the friction of the surface on which the body rests, is necessarily 

 introduced into our calculations. It will depend on the nature of the surface over which the 

 transport takes place, and on the force with which the body presses on that surface, and this force 

 will depend very much on the extent of that portion of the surface of the body whicli may be 

 in such close contact with the surface on which the body reposes as to exclude the water from 

 penetrating between them, and exercising there its upward pressure. In those cases, however, 

 in which the motion takes place by rolling, the uncertainty depending on friction is entirely 

 removed, for such motion is independent of the magnitude of the friction. Also, during a rolling 

 motion the body must be revolving round one edge as an instantaneous axis, so that the fluid 

 pressure will act on all points of the surface except those very near to that axis. The abstraction, 

 therefore, of the pressures on these latter points will have no material eft'ect on the body's rolling 

 motion, and may be neglected in our calculations. When the body passes from one edge to 

 another, as a new instantaneous axis, the whole intervening surface might come in close contact 

 with that over which the body moves ; but if these edges be not too far apart (as will generally 

 be the case in those bodies which tend to move by rolling rather than sliding) the body will 

 necessarily begin, by its momentum to move round the second axis, and will conscciuently admit 

 the fluid to exert its pressure on the lower surface of the body, after it has passed to a new 

 axis of instantaneous rotation. 



