84 ELEMENTS OF STATIC ELECTRICITY. 



till the force of the original impulse has been exhausted, 

 and the water returns to its original level. 



Now it will be perceived that there is no transfer of 

 the water from the center outward beyond the length of 

 the first wave. Part of the water forced outward by the 

 original impulse flows back again, while another part 

 flows outward, producing a new wave. The water is then 

 under the influence of two forces, one horizontal, the 

 other vertical, acting at right angles to each other; the 

 horizontal producing the wave length, that is, the 

 distance from crest to crest, or from hollow to hollow, 

 while the vertical produces the height, that is, the 

 vertical distance from hollow to crest, or from crest to 

 hollow. 



In a similar way, it is supposed, occur the undula- 

 tions of the assumed elastic medium, with this excep- 

 tion, that the waves on the water occur in the same 

 horizontal plane, radiating outward in concentric cir- 

 cles, while those in the elastic medium occur in any 

 direction in which they are free to move ; radiating 

 outward in concentric spheres, if wholly unrestrained ^ 

 or in sections of spheres or spheroids, if limited and 

 starting from impulses at various points on any surface, 

 either spherical, like that of the sun, or plane. 



Having taken an illustration from a liquid, illus- 

 trations from solids will also be in point. 



If a long rope, stretched lengthwise, with plenty 

 of slack, be held at one end, and jerked rapidly up and 

 dewn, it will be thrown into w r aves, which will run 

 along its entire length. 



Here it is evident that while the impulses given at 

 one end travel in waves to the other, the rope, as a 

 whole, remains stationary ; successive portions acting 



