1831.] on Vibrating Elastic Surfaces. 355 



ridges in hot oil, but not in cokl 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 ot crispation produced 

 when a large glass full of water is made to sound by passing 

 the wet ringer 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 described. If, during a strong 

 steady wind, a smooth flat sandy shore, with enough water on 

 it, either from the receding tide or from the shingles above, to 

 cover it thoroughly, but not to form waves, be observed in a 

 place where the wind is not broken by pits or stones, stationary 

 undulations will be seen over the whole of the wet surface, 

 forming ridges like those already described, and each several 

 inches long. These are not waves of the ordinary kind ; they 

 are accurately parallel to the course of the wind 5 they are of 

 uniform width whatever the extent of surface, varying in width 

 only as the force of the wind and the depth of the stratum of 

 water vary. They may be seen at the windward side of the 

 pools on the sand, but break up so soon as waves appear. If 

 the waves be quelled by putting some oil on the water to wind- 

 ward, these ripples then appear on those parts. They are often 

 seen (but so confused that their nature could not be gathered 

 from such observations) on pavements, roads, and roofs when 

 sudden gusts of wind occur with rain. The character of 

 these ripples, and their identity with stationary undulations, 

 may be ascertained by exerting the eye and the mind to resolve 



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