THE FORMATION OF SEA-WAVES. 495 



storms. The transmission may be effected at very great distances 

 several hundred miles, for example. It presents a kind of analogy 

 with earthquake-shocks which pass over dead points, and make them- 

 selves felt only in places where there are faults or differences in the 

 density of the terrestrial strata. 



At Havre, on stormy days, the perturbations of the open sea, trans- 

 mitted to the shoals of the roadstead, cause oscillations of the water 

 within it, and produce what the sailors call the levee, which, in bad 

 weather, prevents the transatlantic steamers from entering the port. 

 Crossing the entrance of the port, with its breakwaters, the levee pene- 

 trates into the outer harbor and spreads out there, attaining two or 

 three metres in amplitude. It enters the Bassin du Boi through a 

 sluice sixteen metres in width, and thence is propagated through a 

 sluice thirteen metres wide to the Bassin du Commerce, where, involved 

 in the ins and outs of the quays, it does not reach more than thirty or 

 fifty centimetres in amplitude. This remarkable phenomenon of the 

 levee, passing into a chain of basins, appears analogous to that of the 

 vibrations of a tense cord divided into sections by a series of frets 

 in contact with it. "When we draw a bow over one of the sections 

 of the cord, the others will also vibrate, while a dead point or node 

 will be formed at each place of contact. In the phenomenon under 

 consideration, the entrance of the port and each sluice give rise to 

 a node. 



When a mass of water in motion meets an obstacle, it accumulates 

 against it by virtue of its inertia ; the water rises, then falls back. 

 This is called the surf, and may be observed along all coasts. It is 

 produced at sea after every tide. The most curious effect induced by 

 it is the back-flow in rivers. The Seine, for instance, flows rapidly at 

 low water ; but, as the tide rises, a liquid obstacle several metres high 

 is piled up in less than two hours against the mouth of the river. The 

 water of the Seine then stops, rises, and falls back as surf, while the 

 surf in its turn acts as an obstacle to the current of the river above 

 it. The phenomenon is repeated, and again, and so on, steadily going 

 higher up the river, so that in effect a strong wave ascends the stream. 

 The phenomenon may be easily reproduced on a small scale ; every 

 time we suddenly stop the rapid current of a brook or any stream of 

 water, we may see a back-water ascend it. 



The amplitude of the movement of waves remains to be spoken of. 

 It appears to be proportional for direct waves to the force of the wind. 

 On the other hand, since each ripple, wavelet, or wave, occupies a given 

 space, and since, as I have already said, a certain number of these are 

 necessary to give rise to a billow or a heavy sea, it is evident that, 

 with a given wind, a billow of a particular dimension can be formed 

 only if a sheet of water extends over a certain area of surface. This 

 is precisely what takes place, and the dimensions of the billows pro- 

 duced by a given wind appear to be proportional to the extent of the 



