OF FLUIDS ON VIBRATING ELASTIC SURFACES. 
337 
of the cylinder immersed, the ridges diminished in strength, and at last dis- 
appeared : when the cylinder of cork just touched the surface, they were most 
powerfully developed. This is a necessary consequence of the dependence of 
the ridges upon the portion of water which is vertically displaced and restored 
at each vibration. When that, being partial in relation to the whole surface, is 
at or near the surface, the ridges are freely formed in the immediate vicinity ; 
when at a greater depth (being always at the bottom of the cork), the dis- 
placement is diffused over a larger mass and surface, each particle moves 
through less space and with less velocity, and consequently the vibrations must 
be stronger or the ridges be weaker or disappear altogether. The refraction 
of a light through this star produces a very beautiful figure on a screen. 
120. A heavy tuning-fork vibrating, but not too strongly, if placed with the 
end of one limb either vertical, inclined, or in any other position, just touching 
the surface of water, ink, milk, &c. (75), shows the effect very well for a mo- 
ment. It also shows the ridges on mercury, but the motion and resistance of 
so dense a body quickly bring the fork to rest. It formed ridges in hot oil, but 
not in cold oil (76). With cold oil a very inclined fork produced a curious 
pump-like action, throwing up four streams, easily explained when witnessed, 
but not so closely connected with the present phenomena as to require more 
notice here. 
121. There is a well known effect of crispation produced when a large glass 
full of water is made to sound by passing the wet finger round the edges. 
The glass divides into four vibrating parts opposite to which the crispations 
are strongest, and there are four nodal points considered in relation to a hori- 
zontal section, at equal distances from each other, the finger always touching at 
one of them. If the vessel is a large glass jar, and soft sounds are produced, the 
surface of the water exhibits the ridges at the centres of vibration ; as the 
sound is rendered louder, these extend all round the glass, and at last break 
up at the centres of vibration into irregular crispations, but both the ridges 
and crispations are effects of the kind already described, and require no further 
explanation. 
122. There are some other effects, one of which I wish here briefly to notice, 
as connected more or less with the vibratory phenomena that have been de- 
scribed. If, during a strong steady wind, a smooth flat sandy shore, with 
2x2 
