WAVES BETWEEN OIL AND WATER. £37 



ployed to disturb its equilibrium, this will take place toacoh- 



siderable degree, in a time when it would have been scarcely 



perceptible had there been but one liquid in the vessel. 



Thus, in the instance stated by Franklin, the swinging of as observad bj 



the glass produces scarcely any agitation at the surface of tranklin ' 



>theoil ; because, though the glass inclines alternately to each 



side, as the motion is moderate, the surface of the oil returns 



to an equilibrium as fast as it is diverted from it. But the 



surface of the water, having to restore its equilibrium only 



the excess of its weight over that of the oil, which is very 



trifling, as we may estimate it at about 0*006 of the weight 



of the water, allows !he small deviations from the level time 



to accumulate; so that this surface is no longer level when 



the swinging ceases, and is obliged to return to it by very 



slow and considerable undulations, that continue a long 



time 



Of all the modes, in which this phenomenon may be varied, The exp«ri. 

 I shall mention but one, where the cause I have mentioned ment vaned - 

 is too obvious to be mistaken. 



Take a glass globe, mounted so as to be capable of being 

 turned on its axis; put into it water alone, so as to fill it to 

 a quarter of its diameter; and turn it gently : the water will 

 continue apparently to occupy the lower part of the globe. 

 Fill it then to three fourths, and the appearance will be the 

 same, if you turn it in the same manner. So it will if you 

 put oil alone, instead of water. Lastly, pour in water to one 

 fourth of its height, r.nd upon this oil to three fourths: then, 

 if you turn the globe, there will be no change at the surface 

 of the oil, and the whole body of liquid will appear to occupy 

 the lower part of the globe. But with the water it will be 

 different. When you have turned the globe a quarter round, 

 you will perceive it nearly at the extremity of the horizontal 

 diameter, instead of being in the lower part of the globe: 

 and, if you then stop the rotary motion, the water will de- 

 scend slowly down the side of the globe to the lower part, 

 will ascend on the other side nearly to the same height, and 

 thus oscillate a long time, till it settles at its lower part. 



As I have said above we here see clearly, that the particu- 

 lar motion of the water under the oil has the particular cha- 

 racter of that of solid bodies in fluids ; and as it is the phe- 



nomeaoo 



