3(5: COIiESlON^ OF FLUIDS. 



On the nppa- on the actual ascent of water. The diffeiencc is much smaU 



tent attractions 2gj. ^j^.^^^ ^j^^ difierence of the various experiinents on the ascent 



and repulsions ' 



offloati/itj of fluids; and it may easily have arisen from a want of per- 



^^^' feet parallelism in the plates ; for there is no force tending to 



preserve this parallelisiru The error, in the extreme case of 

 the plates coming into contact at one point, may reduce the 

 apparent cohesion to one half. 



The same theory is sufiicicnt to explain the law of the force 

 by which a drop is attracted towards the junction of two plates 

 -inclined to each other, and which is found to vary in the in- 

 verse ratio of the square of the distance; whence it was infer- 

 red by Newton that the primitive force of cohesion varies in 

 the simple inverse ratio of the distance,, while other experi- 

 * ments lead us to suppose that cohesive forces in general vary- 

 in the direct ratio of the distance. But the difficulty is remov- 

 ed by considering the state of the marginal surface of the drop. 

 If the plates were parallel, the capillary action would be equal 

 on both sides of the drop : but when they are inclined, the 

 curvature of the surface at the tliinncst part requires a force 

 proportionate to the appropriate height to counteract it ; and 

 this force is greater than that which acts on the opposite side. 

 But if the two plates are inclined to the horizon, the defi- 

 ciency may be made up by the hydrostatic vi'eight of the 

 drop itself; and the same inclinatioa will serve for a larger 

 or a smaller drop at the same place. Now when the drop 

 approaches to the line of contact, the difference of the 

 appropriate heights for a small drop of a given diameter will 

 increase as the square of the distance decreases ; for the fluxion 

 of the reciprocal of any quantity varies inversely as the square, 

 of that quantity : and, in order to preserve the equilibrium, 

 the sine of the angle of elevation of the two plates must 

 be nearly in the inverse ratio of the square of the distance 

 <if the drop from the line of contact, as it actually appears 

 to have been in liauksbee's experiments. 



VI. Fhyskal Foundation of the Lave of svperficial Cohesion. 



Law of superll- We have now examined tiie principal phenomena which are 



reducible to the simple theory of the action of the superficial 



particlfsofa fluid. We are next to investigate the natural 



foundations upon which that theory appeals ultimately at rest. 



We 



