208 



HYDRODYNAMICS. 



quired, and breaks into foam and lashes the earth and 

 rocks. The sea-billows are sometimes twenty-five feet 

 in elevation,* and when these advance upon a stranded 

 ship on a lee shore, with the speed of a locomotive, 

 their effects are in the highest degree appalling, and 

 iron bolts are snapped and massive timbers crushed 

 beneath their violence. 



PREVENTING THE INROAD OF WAVES. 



To prevent the inroads of lake waves upon land, the 

 remedies must vary with circumstances. The diffi- 

 culty would be small if the water always stood at the 

 same height. The greatest mischief is usually done 

 when they rise over the beach of sand and gravel which 

 they have beaten for centuries. Wooden bulwarks soon 

 decay. Wliere loose stone can be had in large quan- 

 tities they may be cheapest, but they are not unfre- 

 quently placed too near low- water mark to protect the 

 banks. Substances which offer a gradual impediment 

 to the waves are often quite effectual, though not for- 

 midable in themselves. It is curious to observe how 

 so slender a plant as the bulrush, growing in water 

 several feet deep, will destroy the force of waves. If 

 it grew only near the shore, where the water has pro- 

 gressive motion, it would soon be dashed in heaps on 

 the beach. Parallel hedgerows of the osier willow, 

 protected by a wooden barrier until well grown and 

 established, would in many cases prove efficient. 



Stones and timber bulwarks are often made need- 

 lessly liable to injury by being built nearly perpen- 



* No authentic measurement gives the perpendicular height of 

 ■waves more than twenty-five feet. 



