WAVES BETWEEN OIL AND WATER. Og^ 



the Earth with equal energy; and, approaching it as long as 

 they find no sufficient obstacle in the adjacent columns, do 

 not stop till they arrive at that state, which is called the 

 level. And as this motion is an effect of gravity, it is acce- 

 lerated, carries all the parts beyond the point of equilibrium, 

 and causes them to vibrate several times round this point, 

 producing undulations, a kind of oscillations with which 

 every one is familiar. 



By the second principle, a body moving in a liquid, being Motion of a 

 obliged, in displacing it, to communicate to it continually a n 0( ?J' aa 

 part of its motion, is incessantly losing force; so that, in 

 obeying the laws of gravitation, it falls through the liquid 

 only with the excess of its specific gravity over'that of the 

 liquid in which it moves. 



It might be supposed at first view, that this second priori- Application of 

 pie should have no influence over either of the two liquids, these P«a«- 

 that exhibit the phenomenon in question, because neither of** 

 them is properly in the other. Both, however, are subjected 

 to this law : the lowermost, because its surface cannot acquire 

 any undulatory movement without displacing the upper; 

 and the uppermost, because its snrfacecannot move without 

 raising the air, which then presses on it at every point. 

 But, as we are accustomed to see the effects of this position 

 with regard to the air, we do not think of referring them to 

 their cause. 



With respect to the inferior liquid, its situation and re- 

 lation to the superior render a phenomenon very remarkable, 

 which at the same time is essentially the same with what we 

 see without attention at the surface of the superior. 



To understand the reason of this singularity, let us sup- Change of 



pose some cause to have disturbed the surface of the inferior ! eve ' in , a 0uii 

 ;. . , , . . , , , t • i • beneath ano- 



liquid, so that it is no longer level, but a given column is a ther not much 



certain degree higher than another. This column does not 1, g hter « 



exceed the shorter in weight by the whole quantity it is 



higher, for it makes only a part of the column that exists 



in the vessel at that point, consisting of the heavier liquid at 



its lower part, and the lighter at its upper. The shorter 



column of the inferior liquid in like manner is only the lower 



part of a column, the upper part of which is formed of the 



lighter liquid. The difference between these two columns is, 



the 



