Illustration from a river. 



103 



I have given, in the annexed figure, a sketch of what occurs 

 in the course of the river Rhone at the bridge near Sierre. 



The course of the current is indicated by the arrows, the pier 

 of the bridge b embays the water at c, where it is whirled about 

 by the tangential action of the current but does not escape, 

 the wavy lines e d indicate the ripple produced by the friction 

 of the central past the lateral portions of the stream, the dis- 

 turbance from which is propagated towards the shore in the 

 waves/. All that is here described may be still better seen 

 in a sluggish stream, and if it be covered with any kind of 

 scum, it will be torn up into longitudinal shreds between e 

 and d exactly corresponding to the position of the icy bands. 

 The nature of the force producing these lines of scum is also 

 well shewn by the rotatory motion acquired by any floating 

 patches of foam, which is always in a direction forwards 

 and towards the side of the stream to which it is nearest ; 

 thus, in the preceding figure, it would be in the direction of 

 the hands of a watch. I remain, my dear Sir, yours very 

 truly, 



James D. Forbes. 



Edinburgh, 24«A October 1846. 



Erratum. — In the Eleventh Letter, in the Ed. Phil. Journal for October, 

 p. 421, line 18, 



for V 9 + (9 cotan i — p cosec 6} "^ 



read V 9* + (9 cotan ^ — ;' coeec 0) " 



