WAVES. 4' 



A column of water having a velocity of 41 feet per second, 

 as in the case of the storm waves referred to on p. 30, is such as 

 would be produced by a 26-feet head of water. It would there- 

 fore exert a pressure of only 1622 Ibs. per square foot; whereas 

 the dynamical force of storm waves has been registered at as 

 much as 6083 Ibs., or nearly 3 tons per square foot. 1 



Examples of the effect and power of waves will be found in 

 the chapter upon " Exposure," p. 73. 



Wave-action on Beaches. It is well known that off-shore 

 winds cause beaches to heap up, while on-shore gales draw them 

 down ; or, as I have heard fishermen say, " the sand works 

 through the wind." 



It would, perhaps, be too much to say that this rule is appli- 

 cable under all circumstances, but it will generally be found 

 correct. 



It has been observed that, under certain conditions, the period 

 of waves may be taken as an indication of the action that is 

 going on. Thus, for instance, seven or any less number of waves 

 per minute would indicate erosive action, whereas nine or any 

 greater number would indicate an accumulative action. 2 



Although, under the conditions existing where these observa- 

 tions were made, the period of the waves was doubtless a fairly 

 reliable indication of the action that was going on, I think there 

 are many cases where such a rule would not apply. 



Sandy shores, exposed to the constant long roll from the 

 ocean, will be found (at least, such is my experience) to alter- 

 nately accumulate and be pulled down more in obedience to the 

 direction of the wind than to the period of the waves, and I 

 have noticed that some shingle beaches are affected in the same 

 manner. 



The reason why a change in the direction of the wind 

 should so affect the action of waves upon a beach is not well 

 understood. 



I have on several occasions endeavoured to fathom the 

 mystery by watching the motion of balls of burnt clay, which, 

 owing to their form and comparatively small specific gravity, 

 were easily moved by the waves ; but, I must admit, the results 

 of these observations were not such as to enable me to base any 

 theory upon them. 



1 Trans. Royal Society of Edinburgh, vol. xvi. 



2 "Coode's Description of the Chesil Bank," Min. Proc. InsL C.E., vol xii. 



