IN a letter published in NATURE on August 19, 1920, p. 777, I said that, judging from appearances, wind-formed waves increased in length only when the waves were in a “breaking” condition, and it was suggested that the breaking crests, by imparting their momentum to the upper strata of the water, and so establishing a gradually increasing surface current, would necessarily also cause a gradual increase in the wavelength equal to the increase of stream speed per wave-length × by the period of the wave. An example of the alteration in the lengths of a train of breaking waves may be seen at the foot of any weir where the flow is sufficient to form a stationary “bore.” The wave where the stream over the apron is first arrested carries a large breaking crest, and down-stream follow several waves also breaking, but not so heavily. Following these again are waves which do not break, and counting from the first wave, observation will show that the distance from crest to crest diminishes notably so long as the waves are breaking.