Hunt — On the Action of Waves on 8ea-heaches, 8fc. 253 



The assertion that is made is sufficiently startling, as it is in 

 effect that a wave of oscillation that propagates itself by means of 

 a pendulum-like vibration about the level of repose of the water 

 can be raised entirely above that level, and there sustained, without 

 the intervention of any force outside itself. 



Mr Russell, in his first report to the British Association, describes 

 an experiment made with marine waves, in the following passage : — 

 " The phenomena of waves breaking on the shore were observed 

 principally on a very fine smooth beach of sand, having a slope 

 towards the sea of 1' in 50* ; so perfectly plane and level was it at 

 the time when the observations were made, that a single wave a 

 mile in breadth might be observed advancing to the shore, so per- 

 fectly parallel to the edge of the water that the whole wave rose, 

 became cusped, and broke at the same instant ; a line of graduated 

 rods was fixed in the water at different depths from 6 inches to 

 6 feet in length, and it was observed that every wave broke exactly 

 when its height above the antecedent holloio was equal to the depth 

 of the water."— (Tm^is. Brii. Assoc, 1837, p. 450.) The words I 

 have italicised, if taken in their literal sense, would be sufficient to 

 settle the question at issue, for waves with antecedent hollows, by 

 Mr. Eussell's own definition, are not waves of the first order, as the 

 latter have their surfaces "wholly raised above the level of repose of 

 the fluid." Sir George Airy, commenting on this passage {Tides and 

 Waves, 403), remarks that Mr. E-ussell does not state " whether this 

 depth was measured from the mean level of the surface, or from the 

 bottom of the hollow,'' and thus clearly assumes that the *' bottom 

 of the hollow" and Mr. Russell's "antecedent hollow" are not 

 equivalent to the level of repose. Mr. Russell's meaning is, how- 

 ever, open to some doubt, as on a subsequent occasion he defined 

 " the limit of height of a wave of the first order " as " a height 

 above the bottom of the channel equal to double the depth of the 

 water in repose." — {Trans. Brit. Assoc, 1844, p. 352.)^ 



There can, however, be little doubt that waves derived from 

 waves of oscillation under the circumstances described by Mr. 



^ The question is whetter, in the passage referred to, Mr. Eussell uses the -words 

 " antecedent hollow " in their ordinary sense as descriptive of a depression extending 

 below the level of repose of the water, or as descriptive of the depression between two 

 waves of his first order, which depression, according to his own definition, must not (as 

 part of a wave of the first order) extend below the said level of repose. 



