1899.] The Motion of a Perfect Liquid. 49 



WEEKLY EVENING MEETING, 



Friday, February 10, 1899. 



The Hon. Sir James Stirling, M.A. LL.D., Vice-President, 

 in the Chair. 



Pr»fes8ob H. S. Het.e-Siiaw, LL.D. M. Inst. C.E. 



The Motion of a Perfect Liquid. 



If we look across the surface of a river, we cannot fail to observo 

 the difference of the movement at various points. Near one bank 

 the velocity may be much less than near the other, and generally, 

 though not always, it is greater in the middle than near either bank. 

 If we could look beneath the surface and see what was going on 

 there, we should find that the velocity was not so great near the 

 bottom as at the top, and was scarcely the rame at any two points of 

 the depth. The more we study the matter, the more complex the 

 motion appears to be ; small floating bodies are not only carried 

 down at different speeds and across each other's paths, but are 

 whirled round and round in small whirlpools, sometimes even disap- 

 pearing for a time beneath the surface. By watching floating bodies 

 we can sometimes realise these complex movements, but they may 

 take place without giving the s-lightest evidence of their existence. 



You are now looking at water flowing through a channel of 

 varying cross section, but there is very little evidence of any dis- 

 turbance taking place. By admitting colour, although its effect is at 

 once visible on the water, it does not help us much to understand the 

 character of the flow. If, however, fine bubbles of air are admitted, 

 we at once perceive (Fig. 1) the tumultuous conditions under which 

 the water is moving and that there is a strong whirlpool action. 

 This may be intensified by closing in two sides (Fig. 2), so as to 

 imitate the action of a sluice gate, through the narrow ojiening of 

 which the water has all to pass, the presence of air making the dis- 

 turbed behaviour of the water very evident. 



Now you will readily admit that it is hopeless to begin to study 

 the flow of the water under such conditions, and we naturally ask, are 

 there not cases in which the action is more simple ? Such would be 

 the case if the water flowed very slowly in a perfectly smooth and 

 parallel river bed, when the particles would follow one another in 

 lines called " stream-lines," and the flow w< uld be like the march of 

 a disciplined army, instead of like the movement of a disorderly 

 crowd, in which, free fights taking place at various points may be 

 supposed to resemble the local disturbances of whirlpools or vortices. 



Vol. XVI. (No. 93.) e 



