Mr. HOPKINS, ON THE TRANSPORT OF ERRATIC BLOCKS. 



and 



2g- V|0, / c 



Let c = na, then 



If the section be square, w = 1, and 



1 



1,5 2g 

 If the body be on the point of sliding, we must have 



— . p-,.bc = fx {p - pi)gabc, 



.,2 



a = 



'te-0 



2^ 



This value of a is less than the former one if 



1 



n 

 If w = 1, or the section be a square, the condition becomes 



If this hold, the body will roll rather than slide. 



15. Let us next take a prism of which the transverse section is a pentagon. 



Let the side of the pentagon = a, and the length of the i' 



prism = h; JCQ = 0, CQ being horizontal i and let RiR^ ^^ 

 the normal forces due to the current on AC, CD respec- 

 tively, c 



To find the tendency of these forces to turn the body 

 round an axis through B and perpendicular to the section, we 

 observe that a perpendicular to CD through its middle point 

 would pass through B, and consequently the moment of R^ 

 round the proposed axis will = 0. Also, since tlie direction of 

 i?, will bisect the angle at E, we shall have, when the prism is on the point of turning about 

 the edge througli B, 



. BED , a 



^1 asm -i;— =(/>-/» )f^fi"- 9 ' 



or, substituting for i?, and U, 



- p,a^b&in^e cos 36" = -{p- pi) g— cot 36". 



G C2 



