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



Therefore (since B = ~2) 



''' _^ (p A '' 



!?£• 8 Vp, j sin'" 7a" sin 3(f ' 



sin=72''sin.'!6'' v^ 



2ff 



8 Vp, J 



= .567—, 



putting — = 2, 5 . 

 P\ 



If we suppose the hody on the point of sliding we find the value of a nearly equal to that 

 just given, supposing n= 1. 



16. Again, let the section of the prism be hexagonal. Let JB = o, and R' be the horizontal 

 force of the current on the side JC = that on the side CD. Then 

 when the body is on the point of turning about the side through " 



B, we shall have 



But 



and 



and 



2R'.H0^(p-p,)gU'- 



R' = — pj ab sin' 60, 



U=3a.H0.b; 



v'^ 3 



2 —p^ab sin' 6o = -(p - p^)ga?h. 



^g 



2ff 



=K-) 



V' 



,57.— 

 2g 



early. 



It will be observed that in all the preceding cases the results are independent of the lengths 

 of the prisms, as they manifestly ought to be, since by changing the length of a prismatic body 

 situated as above supposed, the mass and the force upon it are changed in the same ratio. 



The tendency to roll as compared with that to slide is easily shewn to be greater in this than in 

 the preceding cases ; and if we take a prism of which the section should be a regular polygon 

 of a still greater number of sides, the tendency to roll would be still greater. It is unnecessary 

 to increase the number of examples of this kind ; but there is another case somewhat different 

 from the above which is deserving of notice. 



17- Many of the erratic blocks which may be referred to the agency of currents are so 

 rounded as to approximate more or less to the spherical form. Let APE represent the locus of 

 those points on the surface of the body which come consecutively in contact with the ground in the 



