396 Lord Rayleigh on the Tension of Water 



lying upon it was caused to vibrate by an external magnet*. 

 In making the necessary arrangements there was some risk 

 of introducing contamination, so that the discovery of an 

 unclean surface would prove nothing definite. If, however, 

 the behaviour of the surface under the needle test was good, 

 it could be inferred with confidence that the measured waves 

 were not affected by impurity. On two occasions the test 

 succeeded fairly well. 



The observations with the 128 fork were rather difficult, 

 the waves being about twice as close as in the other case. In 

 the calculation of results it appears, as was to be expected, 

 that the importance of the second term, due to gravity, is 

 diminished. Thus for July 22, 



T=76-5-2-4=74-l. 



The general result that at temperatures such as 65° (18° C.) the 

 tension of clean water surfaces is about 74 "0 C.G-.S. absolute 

 units of force per centimetre seems entitled to considerable 

 confidence. It agrees with some former observations f of my 

 own upon the transverse vibrations of jets, as has been re- 

 marked by Mr. Worthington J. Some interesting experiments 

 upon the vibrations of falling drops by Lenard § point also in 

 the same direction. On the other hand it deviates largely 

 from the higher value, about 81, which Prof. Quincke thinks 

 the most probable. The deviation from 81 is certainly not 

 due to contamination. It has been explained that great care 

 was taken in this respect during the present experiments ; and 

 in the jet method the surfaces are probably the purest attain- 

 able. The method favoured by Quincke depends upon the 

 measurement of large flat bubbles confined under the horizontal 

 surface of a solid body. In default of experience I must leave 

 it to others to judge whether a systematic error due to optical 

 or other causes could enter here. Mr. Worthington contends 

 that some of Quincke's deductions from his measurements 

 require correction for curvature perpendicular to the meri- 

 dianal plane. To this and other criticisms Prof. Quincke has 

 replied || . 



Experimenters upon capillary tubes have generally been led 

 to adopt the lower value, but here the interpretation involves an 

 assumption that the angle of contact 6 is zero. What these 

 measurements give in the first instance is T cos 6 ; so that if 

 0=30°, or thereabouts, the higher value of T is the one really 



* " On the Superficial Viscosity of Water," Proc. Roy. Soc. June 1890, 

 vol. xlviii. p. 139. 



t Proc. Roy. Soc. 1879, vol. xxix. p. 71. 



X Phil. Mag. 1885, vol. xx. p. 51. 



§ Wied. Ann, Bd. xxx. (1887). I| Ibid, xxvii. p. 219 (1886). 



