The Right Hon. Lord Rayleigh 



[March 20, 



S2 



suction was exactly 2 : 1, as indicated by proportional compasses. 

 Thus on Januarv 23, when the temperature of the water was 9° C, 

 the 2 : 1 ratio occurred on four trials at 120, 1:^0, 12:^, 126, mean 

 125 mm. head. The temperature was then raised with precaution 

 hv pourin<r in warm water with passages backwards and forwards. 

 The occurrence of the 2 : 1 ratio was now much retarded, the mean 

 head l>einir onlv 35 mm., corresponding to a mean temperature of 

 37^ C. The ratio of head to suction is thus dependent upon the 

 hejid or velocity, but when the velocity is altered the original ratio 

 may be recovered if at the same time we make a suitable alteration 

 of viscosity. 



And the required alteration of viscosity is about what might have 

 been expected. From Landolt's tables I tind that for 9" C. the 

 viscosity of water is '0136^, while for 37° C. it is '00704. The 

 ratio of viscosities is accordingly 1-943. The ratio of heads is 

 125 : 35. The ratio of velocAties is the square-root of this or 1-890, 

 in sutticientiv irood aofreement with the ratio of viscosities. 



Fig. 11. 



In some other trials the ratio of heads exceeded a little the ratio of 

 viscosities. It is not pretended that the method would be an accurate 

 one for the comparison of viscosities. The change in the ratio of 

 head to suction is rather slow, and the measurement is usually some- 

 what prejudiced by unsteadiness in the suction manometer. Possibly 

 V)etter lesults would be obtained in more elaborate observations by 

 several persons, the head and suction being recorded separately and 

 referred to a time scale so as to facilitate interpolation. But as they 

 stiind the results suffice for my purpose, showing directly and con- 

 clusively the influence of viscosity as compensating a change in the 

 velocity. 



In conclusion. I must touch briefly upon a part of the subject 

 where theory is still at fault, and I will limit myself to the simplest 

 case of all — the uniform shearing motion of a viscous fluid between 

 two parallel walls, one of which is at rest, while the other moves 

 tangentially with uniform velocity. It is easy to prove that a 

 uniform shearing motion of the fluid satisfies the dynamical equa- 



