430 Lord Rayleigh on the Resistance of Fluids. 



incut could be tried than the following. Take a piece of cop- 

 per wire and cover with suboxide by gently heating over a 

 Pmnsen burner. Fuse the covered wire into a capillary glass 

 tube. A scarlet glass is thus obtained which is black whilst 

 hot, i. e. exhibits continuous absorption (1), and scarlet whilst 

 cold, exhibiting partial absorption (2). 2 and 3 are bridged 

 over by a host of bodies like ZnO. When hot they, as a rule, 

 exhibit absorption at the blue end of the spectrum (2), but 

 when cold they are white or colourless (3). 



Respecting the merging of Class I. into Class II. nothing 

 much at present can be said, since questions are involved 

 which require further investigation. 



LIII. On the Resistance of Fluids. 

 By Lord Rayleigh, F.R.S.* 



[Plate V.] 



THERE is no part of hydrodynamics more perplexing to 

 the student than that which treats of the resistance of 

 fluids. According to one school of writers, a body exposed to 

 a stream of perfect fluid would experience no resultant force 

 at all, any augmentation of pressure on its face due to the 

 stream being compensated by equal and opposite pressures on 

 its rear. And indeed it is a rigorous consequence of the 

 usual hypotheses of perfect fluidity and of the continuity of 

 the motion, that the resultant of the fluid pressures reduces to 

 a couple tending to turn the broader face of the body towards 

 the stream. On the other hand, it is well known that in prac- 

 tice an obstacle does experience a force tending to carry it 

 down stream, and of magnitude too great to be the direct 

 effect of friction ; while in many of the treatises calculations 

 of resistances are given leading to results depending on the 

 inertia of the fluid without any reference to friction. 



It was Helmholtz Avho first pointed out that there is nothing 

 in the nature of a perfect fluid to forbid a finite slipping be- 

 tween contiguous layers, and that the possibility of such an 

 occurrence is not taken into account in the common mathe- 

 matical theory, which makes the fluid flow according to the 

 same laws as determine the motion of electricity in uniform 

 conductors. Moreover the electrical law of flow (as it may 

 be called for brevity) would make the velocity infinite at every 

 sharp edge encountered by the fluid ; and this would require a 

 negative pressure of infinite magnitude. It is no answer to 



* Communicated by the Author, having been communicated in sub- 

 stance to the British Association at Glasgow. 



