ON WAVES. 375 



Section III. — Waves of the Third Order. 



Capillary Waves. 



Character Gregarious. 



ir ■ . . f Forced. 



Varieties < t- 



[ rree. 



Instances j Dentate waves. 



[ Zephyral waves. 



Capillary Waves. — If the point of a slender rod or wire, being wet,, be 

 inserted in a reservoir of water perfectly still, to a minute depth, say one- 

 tenth part of an inch below the surface of repose, it is known that the surface 

 of the water will visibly rise in the vicinity of this wire, being highest in the 

 immediate vicinity of the wire, and gradually diminishing until it cease to be 

 sensible. I have examined this elevation by reflected rays from the surface, 

 and I find that this elevated mass does not sensibly rise from the surface at 

 more than an inch distance from the centre of the rod, the rod itself being 

 one-sixteenth of an inch in diameter. 



This statical phaenomenon belongs to a well-known class of phaenomena, 

 which have been experimentally examined by many philosophers, and success- 

 fully explained by Dr. Thomas Young and Laplace, and recently investigated 

 very fully and completely by M. Poisson, in his profound work entitled, ' Nou- 

 velle Theorie de I'Action Capillaire,' Paris, 1 831 . An admirable Report on the 

 present state of our knowledge of the phaenomena of capillar}^ attraction will 

 be found in the Transactions of the British Association, vol. ii. All that it is 

 necessary for my present purpose to advert to on this subject is, that the phae- 

 nomena of elevation of fluids by capillary attraction, are chiefly due to the 

 condition of tension of the supei-iicial particles of the water under the influence 

 of a force acting on these supei-ficial particles at insensible distances only, or 

 by physical contact or adhesion. These superficial particles form a chain, or 

 catenary, or lintearian curve, one end supported by the immediate adhesion 

 of one extremity to the solid body at a given height above the water, the 

 other end lying on the surface of the water, the underlying particles being 

 suspended immediately by their mutual adhesion to this superficial film. 

 M. Poisson especially has shown that " capillary phaenomena are due to mole- 

 cular action, modified by a particular state of compression of the fluid at its 

 superficies." I have been thus particular for the purpose not only of ex- 

 plaining my meaning in a future article, but also to justify a term which I 

 am desirous of introducing here as an expression not only convenient, but 

 also philosophically sound. I have called the phaenomena noticed in this 

 section Capillary Waves, because they appear to me to present themselves 

 exclusively in the thin superficial film which forms the bounding surface of 

 the free liquid, and which is already recognised in the known hydrostatical 

 phaenomena of capillary attraction, and which if I may be allowed, I will call 

 the capillary film. 



By capillary waves 1 therefore designate a class of hydrodynamical phae- 

 nomena, which exhibit themselves when particles of water are put in motion 

 under the action of such forces as when at rest produce the usual hydrosta- 

 tical capillary phaenomena. Let the slender rod already alluded to, as sup- 

 porting a capillary column, bounded by a concave surface of revolution, be 

 moved horizontally along the surface of the fluid with a velocity of one foot 

 per second, and we shall have exhibited to us all the beautiful phaenomena re- 

 presented in Plate LVII. In order to produce these phaenomena, it is only 

 necessary that the slender rod touch the surface without descending to any 



