VII 



OF BENARD'S EXPERIMENT 



505 



Ringing the changes on Benard's experiment, we may use unstable 

 layers of various liquids or gases, or even a thin layer of smoke 

 between a hot plate and a cold (Fig. 182). The smoke will form waves 

 and folds and rolHng clouds; then, with increasing and more and 

 more symmetrical instabihty, polygonal or hexagonal prisms; and 

 all these configurations we may deform or "shear" by sliding one 

 plate over the other. Famihar cloud-patterns, as of a dappled or 

 mackerel sky, can be imitated in this way. When the hexagonal 

 prisms have been developed it is found that a steady shear deforms 

 them into a well-known curvihnear tesselated pattern (Fig. 183) 



Direction of 

 motion 



Fig. 



183. 



of air or 



Shear-patterns in an unstable layer 

 raoke. After Graham. 



Fig. 184. Ambulaeral plates 

 of a sea-urchin (Lepides- 

 thes), to illustrate the 

 "shearing" of a pattern 

 normally hexagonal. 

 After Hawkins*. 



and this may be sheared again into hexagons, oriented in an opposite 

 direction to the first t. The very same pattern occurs now and then 

 in organisms, as a deformation of what is normally a pattern of 

 hexagons (Fig. 184). 



* From H. L. Hawkins, Phil. Trans. (B), ccix, p. 383, 1920. 



t Cf. A. Graham, Shear patterns in an unstable layer of air, Phil. Trans. (A), 

 No. 714, 1933; Gilbert Walker and Phillips, Q.J.R. Met. ;Soc. Lvm, p. 23, 1932; 

 Mais, Beitr. Phys. frei. Atmosph. xvii, p. \o, 1930; H. Jeffreys, Proc. R.S. (A), 

 cxvm, p. 195, 1928; Krishna Chandra, ibid, clxiv, pp. 231-242, 1938. After a 

 steady state is reached it is found that in air or smoke the centre of each 

 polygon is a funnel of descent, but it is an ascending column if the layer be 

 liquid. Now the viscosity of a gas increases, and that of a hquid decreases, with 

 rise of temperature, and the greater the viscosity the more stable is the layer. 

 Accordingly, for a gas the upper, and for a liquid the lower layer is the more 

 unstable, and it is there that in each case the flow begins. 



