62 



THE MADRAS HARBOT T R. 



Now, it will be seen that, in undertaking to build an 

 upright wall in the sea, the engineer has to determine certain 

 points which in the case of a slope the sea may determine for 

 him. If he throws a heap of stone into the sea it stands at 

 first at a steep slope, but the waves soon flatten this down, 

 drawing out the foot and cutting away the top, and if the 

 slope so made does not effect the required purpose, he simply 

 adds more stone till he gets what he wants. 



But in the case of the upright wall, the sea gives him no 

 help except by guiding him to that wisdom which is the 

 result of experience. If Father Neptune does not like his 

 first wall he simply washes it down and leaves the poor 

 engineer to find out where the fault was which he must 

 avoid in his next attempt. This has been done so repeatedly 

 that the lesson has been pretty well impressed upon us in 

 some respects. What Neptune insists upon most uncompro- 

 misingly is an ample depth of foundation. If the natural 

 foundation is solid rock we may build upon it with materials 

 as solid as the rock itself, and Neptune is satisfied. But on 

 the Madras shore there is no solid rock, there is nothing but 

 sand, and in such a case the only security consists in depth. 



It is found by experience that the waves have their great- 

 est force near the surface. At a depth below the surface, 

 which varies with the height of the waves, there is no motion 

 in the water. At lesser depths there is motion, sufficient 

 near the surface to roll over masses of stone of many tons 

 weight, sufficient at a few feet depth to disturb rubble stone 

 in smaller masses, sufficient at a still greater depth to disturb 

 sand, and at one still greater mud. Each material is stable 

 at a certain slope above these levels if the wave runs freely 

 up and down it, but if any abrupt obstacle is placed upon 

 the slope the motion of the wave is interfered with, and it 

 resents the interference by tearing up the material upon 

 which the obstacle is placed. 



