t 441 ] 



XLIII. Liquid Friction. By John Perry, F.R.S., assisted 

 by J. Graham, B.A., and C. W. Heath*. 



[Plate VII.] 



APIECE of apparatus such as is used in this investi- 

 gation was designed and partly constructed in Japan 

 in 1876 ; it is described in my book on Practical Mechanics 

 (1883). The specimen actually used by us was constructed 

 at the Finsbury Technical College in 1882 ? and has been 

 occasionally used since that time, but no complete sets of 

 observations were attempted till October 1891. 



The simplest hydro dynamical condition of viscous fluid is 

 that of the fluid bounded by two infinite parallel planes, the 

 fluid in one boundary being at rest, the velocity in the other 

 boundary being constant and in the plane. Motions in a 

 pipe and near a vibrating disk, or even near a steadily 

 rotating disk, are rather complicated. Our apparatus was 

 designed to approach as nearly as possible to the conditions 

 subsisting between the infinite planes. Between two such 

 planes, if V is the constant velocity of one of them, the other 

 being at rest, and b is their distance asunder, the fluid being 

 of uniform density, and gravity being neglected, if z is 

 measured at right angles from the fixed plane, the equation of 

 steady motion is 



S = °> a> 



or 



v = Yz/b; (2) 



and the tractive force per unit area required at the moving 

 plane to maintain the motion is fiY/b, where fi is the co- 

 efficient of viscosity. We have used, instead of planes, 

 concentric cylindric surfaces of as large radii and as small 

 difference in radii as could be conveniently constructed and 

 used (see PI. VII. fig. 2). 



E E E is a cylindric trough, of which the curved parts 

 E and E are brass. The inner and outer radii of this trough 

 are 10*39 and 12'65 centimetres. C, which forms the bottom, 

 is of iron ; and the whole trough can be rotated about its 

 vertical axis at any desired speed by driving the pulley P from 

 a coned pulley D with numerous steps. 



G is a hollow brass cylinder supported by a steel wire L, 

 of 0*037 centim. diameter, 67*78 centim. long, whose axis 

 coincides with the axis of the trough and the axis of 



* Communicated by the Physical Society : read March 24, 189o. 



