100 ON" A SMALL CAPILLAET WAVE 



at a. distance of about two inches from the wave. Every moment 

 brought down fresh accessions, which gradually pushed on before 

 them that which had already arrived, till the whole space soon be- 

 came covered as before ; but there was always left between the 

 wave and the froth the narrow strip of clear water already men- 

 tioned. 



Even here it was not easy to ascertain accurately the shape of the 

 wave, in consequence of its constantly shifting with the undulations 

 and irregularities of the current, but I came to the conclusion that 

 it could rarely be more than -^ of an inch high, and I satisfied my- 

 self, from the distortion of objects seen by reflection, that the wave 

 was convex towards the direction of the stream, and concave towards 

 the log, the sensible convexity not extending so much as half an inch 

 beyond the sharply defined cusp, and the concavity not very much 

 more. It became questionable even whether it could be said strictly 

 to be elevated above the surface at all, not only because, if the far- 

 ther side were convex, it was difficult to conceive how it could regain 

 the level without a corresponding concavity, of which I saw no sign, 

 but also from the consideration of the third form in which the wave 

 may be met with, and which I shall mention presently. I was rather 

 led to conclude that the concave side was depressed below the general 

 level, and that the rise towards the log was very gradual. 



I had subsequently an unusually good opportunity of seeing 

 the wave in a very extreme case, where a boom had been stretched 

 across a current running at least six miles an hour. Here it ap- 

 proached sometimes within two inches of the dense mass of rubbish 

 collected on the boom, but always with a perfectly clear space in- 

 tervening, though much narrower than before. The wave itself 

 was apparently fully a quarter of an inch high, and clearly concave 

 towards the boom and convex beyond ; but the question of whether 

 it was the result of an elevation or of a depression of the general 

 surface could not be decided, because the whole mass of water was 

 heaped up against the boom, and the farther slope of the wave was 

 broken into a succession of ripples, very much exceeding the primary 

 wave both in height and amplitude, and differing from it in not being 

 ctisped, though otherwise imitating its general shape. 



There is one point in which the wave, formed above an impedi- 

 ment in a stream, differs from that seen in advance of a body pro- 

 pelled through still water. At a very low velocity I stated that I had 

 seen it at as great a distance as eight or nine feet from the canoe ; 

 but in the gentlest stream I do not think I ever saw it as far as four 



