S60 ON THE MOTION OF WAVES. 



generally are in the shape of straight furrows, because the wind 

 blows upon the water in a parallel manner, at least for a long ap- 

 parent tract. 



When the water receives several impulses at the same time, but 

 in different places, then the waves which proceed from those places 

 must necessarily cross each other. By this crossing the waves do 

 not disturb each other; but they follow their proper directions, by 

 passing one upon the other. Thus if two stones be thrown upon 

 the surface of stagnant water nearly at the same time, but at a 

 little distance from each other; the circular waves which proceed 

 from those places will be clearly perceived to cross each other, and 

 to follow their peculiar courses. The reason of which is, that the 

 same cause which produces the alternate rising and falling of the 

 water upon the surface of otherwise stagnant water, must operate 

 in the same manner, and must produce the like effect on the surface 

 of another wave. 



When a wave meets with an obstacle which is straight and per- 

 pendicular, such as a wall, or a steep bank, then the wave is 

 reflected by it, and the shape of the reflected or retrogade wave, 

 i sthe reverse of what it would have been on the other side of 

 the obstacle, had the obstacle not existed : For the middle part of 

 the curvature must naturally meet the obstacle, and must be re- 

 flected by it first. And since waves will cross without obstructing 

 each other, the reflected waves will proceed from the obstacle, and 

 will expand all round, &c. 



When the bank or obstacle is inclined to the horizon, as is fre- 

 quently the case on the shores of the sea ; then the reflection of the 

 waves is disturbed, and it is often absolutely destroyed by the fru> 

 tion of the water upon the ground. 



If the obstacle be such as to reflect a part only of the wave, such 

 as a stone or a post, which is surrounded by the water ; then the 

 wave will be partly reflected in shapes and directions which differ 

 according to the form and size of the obstacle, whilst the rest of the 

 wave will proceed in its original direction. 



When a hole in an obstacle permits part only of a wave to go 

 through, then circular waves will be formed on the other side of the 

 obstacle, whose centre is the hole ; for in fact those waves owe 

 their origin to the motion of the water in that place only. 



