60 Shingle on the East Coasts of New Zealand, 



and lagoons, the author does not know of any current of 

 much strength on the coasts to which this paper more 

 especially relates. The wave-action, then, is the chief agent 

 concerned in the transport of the shingle along the Ninety- 

 mile Beach, and near Napier. 



20. The power of waves during a heavy sea is sometimes 

 extraordinary. At the Plymouth Breakwater, blocks of 

 stone weighing several tons each have been washed from the 

 sea-slope over to the land side of the work. At Wick, a 

 mass of concrete and stone, weighing nearly 1400 tons, was, 

 in 1872, gradually slewed round by successive strokes of the 

 waves until it was finally removed from its place and 

 deposited inside the pier. During one storm, "two stones, 

 of 8 and 10 tons in weight respectively, had been carried 

 over the parapet and lodged on the roadway of the break- 

 water." During a heavy gale on the west coast of Scotland, 

 in 1829, waves were observed to exert a force of nearly 3 tons 

 per square foot. At the south-east end of the Chesil Bank 

 pebbles have been thrown to a height of 42 feet above high- 

 water level. Numerous other examples might be given, but 

 these will be sufficient to show that a very moderate sea is 

 likely to be sufficient to move pebbles of the size above given 

 and weighing not more than 7 lbs. or so. Professor Rankin e 

 gives a table* showing that large shingle is moved by water 

 having a velocity of 4 feet per second close to the bed, which 

 would ordinarily be equivalent to a surface velocity of, say, 

 5 miles an hour. This velocity might, under favourable 

 conditions, be generated by waves of one foot or so in 

 height. Little is, however, known accurately of the actual 

 power of waves to move stones of given sizes. The author has 

 observed pebbles of about the above weight (say 7 lbs.) not to 

 be moved by waves of 2 to 3 feet in height ; but this might 

 be partly due to a thin edge of the stone being exposed to 

 the water. Again, stones which have resisted the force of 

 several waves in succession may be suddenly removed by a 

 wave apparently no larger than those immediately preceding 

 it 



III. — Action of Waves on a Beach. 



21. The tendency of wave-action is, when not otherwise 

 expressed, referred to in the following remarks on this sub- 

 ject. The actual effect is that due to the resultant of the 



* See Civil Engineering, 9th ed. (1873), p. 708. 



