HYDRAULICS. 153 



How are waves 344. When OHG porlioQ of a liquid is dis- 

 Burfaces form- tui'bed, the dlsturbauce (in consequence of tlie 

 '^^- freedom with which the particles of a liquid 



move upon each other) is communicated to all the other 

 portions, and a wave is formed. This wave propagates 

 itself into the unmoved spaces adjoining, continually en- 

 larging as it goes, and forming a series of undulations. 

 ^ 345. Ordinary sea waves are caused by^he 



What is the .J c C 



ciigin of sea wmd prcssin^; unequally upon the suriace or 



waves ? ■*■ ,^. 



the water, depressmg one part more than an- 

 other : every depression causes a corresponding elevation. 



Where the water is of sufficient depth, waves have only a 



Does the sub- vertical motion, i. e., up and down. Anv fioatin'^ body, as a 

 stance of the i i r . o ji 



wave actually buoy, floating on a wave, is merely elevated and depressed 

 if sta"ionary r alternately; it does not otherwise change its place. The 

 apparent advance of waves in deep water is an ocular decep- 

 tion : the same as when a corkscrew is turned round, the thread, or spiral, 

 appears to move forward. 



346. A wave is a form, not a thing; the form advances, but 

 Why do waves , , ^ , -rr-. , , ■ 



always break not the substance of the wave. U hen, however, a rock rises 



against tha ^q ^\^f^ surface, or the shore bv its shallowness prevents or re- 

 shore ? - ^ 



tards the oscillations of the water, the waves forming in deep 



water are not balanced by the shorter undulations in shoal water, and they 



consequently move forward and form breakers. Thus it is that waves always 



break against the shore, no matter in what direction the wind blows. 



When the shore runs out very shallow for a great extent, the breakers are 

 distinguished by tha name of surf 



On the Atlantic, during a storm, the waves have been observed to rise to 

 a height of about forty-three feet above the hollow occupied by a ship ; tlie 

 total distance between the crests of two large waves being 559 feet, which 

 distance was passed by the wave in about seventeen seconds of time. 



the earth (i. e., the center of figure) than the source is. But if it had not been for the 

 restraining influence of the cohesive force prevailing among the solid particles, it would 

 have been, through the action of the centrifugal force, three miles higher, instead of two 

 and a half It is therefore below the surface of equilibrium, and the water flows south 

 to fill up the proper level. 



The question as to whether the river flows up, or down, depends on the meaning we 

 attach to the words used. If by row>" we mean toward the earth's center of figure, or 

 toward that part nf the earth's surface where the attraction of gravity is the greatest, as 

 at the poles, then the Mississippi runs up hill. If, on the contrary, down means below 

 the surface of equilibrium, and w means above the surface of equilibrium, then the Mis- 

 sissipi flows downward. If the earth were a perfect sphere, and without rot<Uion, tho 

 river would flow northward. A more complete explanation of this subject will be found 

 In a paper read before the American Academy by Prof. Lovcring ia 1S5C, and in the 

 "Annual of Scientific Discovery" for 1S5T, pp. 179—132. 



