392 Lord Rayleigh on the Tension of Water 



In the case of clean water another pair of forks of about 128 



was employed as a check. The number of beats was 184 per 



minute, and 



184- 

 /= 128 -^=124-9. 



The water was contained in a shallow 12" x 10" porcelain 

 dish ; and before commencing observations its surface was 

 purified with the aid of an expansible hoop of thin sheet brass. 

 The width of the hoop is greater than the depth of water, and 

 it is deposited in the dish so as to include the dipper, but 

 otherwise in as contracted a condition as possible. It is then 

 opened out to its maximum area with the effect of attenuating 

 many times the thickness of the greasy film, which no amount 

 of preliminary cleaning seems able to obviate. It not un- 

 frequently happened that the first attempt to get a clean 

 surface was a partial failure, but a repetition of the operation 

 was usually successful. It seems as if impurity attaches itself 

 to the brass so obstinately that only contact with a clean 

 water surface will remove it. 



In the earlier experiments the waves were generated by a 

 dipper of circular section, a closed tube of glass, somewhat 

 dike a test-tube. The measurements were quite satisfactory, 

 but I felt doubts as to a possible influence of curvature upon 

 wavelength. In order to avoid any risk of this kind, and 

 to render the waves straight from the commencement, a 

 straight horizontal edge of glass plate, about 2J inches long, 

 was afterwards substituted, and worked very satisfactorily. 

 It is not necessary or desirable that the dipper should pass 

 in and out of the water. In most cases the vibrations employed 

 were very small, and the edge of the dipper was immersed 

 throughout. 



The purity of the water surface could be judged by the 

 result of the observation of the number of wave-lengths ; the 

 smallest number corresponding to the purest surface. But it 

 ^soon became apparent that a more delicate test was to be 

 found in the general appearance of the wave pattern. Upon 

 a clean surface there is a strong tendency to irregularity, 

 dependent no doubt upon reflexions, which become more 

 important when the propagation is very free. In order to 

 meet this, it was often found necessary to weaken the vibra- 

 tions of the secondary fork, either by putting it more out of 

 tune with the primary, or by shifting its magnet to a less 

 favourable position, or, finally, by shunting the current 

 across. A slight trace of grease would then render itself 

 evident by a damping down of the waves before any change 



