AND OTHER WATER WAVES 341 



clear of the rock, and but slowly breaking up into 

 spray." 



The phenomenon is very noticeable in the com- 

 paratively thin sheet of water which forms the 

 American Falls at Niagara, but the 2o-feet-thick 

 green water at the " Horse -shoe " retains its 

 homogeneity much better. This fact indicates 

 that the partial break-up of a waterfall is not 

 due so much to friction with the air as to other 

 causes, chief among which is, presumably, the 

 dynamically unstable condition of a sheet of water 

 falling under the constant acceleration of gravity. 

 This partial breaking up, which, as Livingstone 

 well indicates, is an early stage of the process by 

 which the water will, if it falls far enough, be 

 finally resolved into spray, must be greatly hastened 

 in the case of a waterfall fed by a stream already 

 markedly intermittent in its motion. I have already 

 described (p. 273) the unsteady motion of the shal- 

 low water in the rapids above the American Falls. 

 The late Professor J . Tyndall says ' of the Falls : 

 " The descent finally resolves itself into a rhythm, 

 the water reaches the bottom of the fall in periodic 

 gushes." And Captain Basil Hall, in the Cave 



1 " Niagara," a discourse delivered at the Royal Institution of 

 Great Britain, April 4, 1873, published in the Fragments of Science 

 (p. 178). 



