500 



Mr. J. B Almy on Accidental 



apart at the nearest point. In the widening space between 

 them two concave wedges are placed in such way as to 

 prevent the formation of eddies or vortices in the part of the 

 liquid to be examined optically. The cylinders are surrounded 

 by a cast-iron chest, which was filled with the liquid to be 

 examined. The axles of the cylinders pass through packings 



Cross-section of Apparatus. Scale 2 : 10. 



to bearings outside ; packings and bearings are adjustable, so 

 as to admit of variation of the distance between the cylinders. 

 A 4-H.P. electric motor furnished power for running the 

 apparatus. The beam of light, from an oxy-hydrogen calcium 

 light, passes through the polarizing Nicol and Lippich of the 

 polariscope, through the liquid between the cylinders, parallel 

 to the axes of the cylinders, and out through the analyser. The 

 polariscope, a " Lippich half-shade " instrument, has an ad- 

 justable Lippich so that the amount of light admitted can be 

 varied according as the liquid is more or less opaque. 



Theory of the Experiment. 



According to the development by Stokes * for the case of 

 two concentric cylinders immersed in a liquid, the inner one 

 rotating, the outer one fixed in position, there is on each 

 element of volume of the liquid between the cylinders a 

 shearing stress, tangential to the cylinder. The magnitude 



* Stokes, Math, and Phys. Papers, i. p. 102 (1880), or Trans, of the 

 Cambridge Soc. viii. (1845). 



