538 THE POPULAR SCIENCE MONTHLY. 



fully twice as great as the landward slope, and possesses some de- 

 cidedly interesting characteristics. In the first place, it is singularly 

 regular, varying but a very few degrees throughout its entire length ; 

 secondly, it is almost exactly the angle that such a mass of pebbles 



SECTION THROUGH A-8 FIG.L 



ROAD 



Fig. 2. 



would take if uninfluenced by the action of the waves. Constant 

 washing has adjusted it precisely as if the pile had been left to its own 

 conditions of equilibrium. 



The general angle of the upper terrace is 29. It has undoubtedly 

 been lessened by travel, and by the fact that it is out of reach of any 

 but the higher waves, and the constant tendency, therefore, is toward 

 undermining it. This terrace is composed of rather large stones, quite 

 unmixed with sand or gravel. 



The second terrace is steeper, 32 nearly. It is exposed to the high 

 tides, and its surface is made up of rather small stones, distributed 

 with great regularity, still unmixed with gravel, but compacted by the 

 waves to a remarkable degree of solidity. 



The lower terrace has nearly the same angle as the middle one, but 

 its composition is very different. The pebbles are of small size, mixed 

 with much coarse gravel and sand, and pounded by the constant wash- 

 ing of every tide into a hard, smooth, regular slope that effectually re- 

 sists any attempt at undermining. 



The general angle of the wall as a whole is within a few minutes 

 of 30, as nearly as could be determined. Owing to the rough instru- 

 ment employed a simple arrangement of plumb-line and semicircular 

 protractor the angles could only be measured to the nearest half de- 

 gree, but the number of measurements taken leaves little doubt as to 

 the accuracy of the result. 



The size of the stones in some parts of the formation may give an 

 idea of the tremendous force of the rushing waves that produced it. 

 Stones of from twenty to thirty pounds in weight are common along 

 the crest of the wall, fifteen feet at least above high-water mark. 

 And not only this, but stones of the same size have been thrown com- 

 pletely over the wall into the road, more than a hundred feet from the 



