376 REPORT — 1844. 



sensible depth ; and the depth to which it descends in no sensible manner 

 affects the phaenomenon. I have called these phaenomena capillary waves. 



Free Capillary Waves. — If the point of a rod sustaining a capillary column 

 be suddenly raised, so as to allow the capillary film to remain without support, 

 it descends and propagates through the capillary film an undulation which dif- 

 fuses itself in every direction circular-wise, in a small group of about half a 

 dozen visible waves which soon become insensible. Or if a very slender silk 

 fibre, stretched horizontally along the surface of the water, be first wetted, and 

 made to sustain a long strip of the capillary film, and then suddenly with- 

 drawn, leaving a ridge of unsupported fluid, waves parallel to this are gene- 

 rated, which remain longer visible, are short and narrow at first, and becoming 

 longer and flatter, at first about a quarter of an inch in amplitude from ridge 

 to ridge, and about half a dozen in number, they become an inch in amplitude 

 about the time when they are last visible ; their longevity does not exceed 

 tivelve or fifteen seconds, and their visible range eight or ten feet. 



These latter are what I designate the free capillary waves ; the former class, 

 shown in Plate LVII., existing under the continued influence of the disturb- 

 ing force, may be called the forced species of this order of wave. As forced 

 waves, and while under the influence of the exciting body, they may appa- 

 rently attain great velocity ; but if the disturbing body be suddenly removed, 

 they immediately expand backwards from the place where they were crowded 

 by the solid point, and becoming all of nearly equal breadth, move forward 

 together as free waves for twelve or fifteen seconds, at a rate of 8^ inches 

 per second. 



Forced Capillary Waves. — I have already stated that if a slender rod 

 or wire, one-sixteenth of an inch in diameter, be inserted, after having been 

 wetted, into water in repose, there will be raised all round this rod a column of 

 fluid by the action of the capillary forces, as indicated at figure 2, Plate LVI. 

 I have stated that this surface may be observed by reflexion to extend on every 

 side about an inch, forming a circular elevation, bounded by a surface of re- 

 volution round the axis of the rod as a centre ; the line which divides the 

 elevated from the level surface being a circle of two inches in diameter. 

 When this rod is moved horizontally along the surface of the fluid, the form 

 of the elevated mass changes; before the disturbing point the extent of 

 elevation diminishes, and the outline of the capillary volume of fluid sustained 

 by the cylinder ceases to be a figure of revolution, becoming distorted as 

 at fig. 3. At a velocity of about eight inches per second, the capillary 

 volume has taken the bifurcate form, fig. 6, and a small wave, b b, about an 

 inch broad, is visible before the disturbing point, and a ridge, a a, begins to 

 manifest itself, diverging from the disturbing body ; at about ten inches per 

 second there become visible distinctly three waves, the disturbing body being 

 in the middle of the first a, and the sum of the length of waves b and c, 

 being about an inch. At higher velocities than this, the waves increase 

 rapidly in number, diminish in amplitude, and extend out in length, spreading 

 into the form indicated in Plate LVII., which is formed at a velocity of 60 feet 

 per minute, or of 12 inches per second. 



As the velocity increases, the following changes are to be observed : — 

 1. The waves diminish in amplitude from ridge to ridge ; that is to say, de- 

 nominating the wave in which is the disturbing body ridge a, and the others 

 in succession before the point bed, &c. the first space of an inch forward, in 

 the direction of motion contains at a velocity of 12 inches per second, or 60 

 feet per minute, besides a, 3 ridges bed; at 65 feet per second, 4< ridges 

 b c de; at 12 feet per second there are in the first inch formed five ridges 

 bcdef, and so on. This crowding of the ridges with the velocity is given 

 in the following Table : — 



