CAPILLAR Y A TTRA CTION. 1 7 



will clearly press most where the convexity is 

 greatest. A very elementary piece of mathematics 

 tells us that on the rigid convex surface which 

 you see, the amount of its pressure per square 

 centimetre will be found by multiplying the sum l 

 of the curvatures in two mutually-perpendicular 

 normal sections, by the amount of the force per 

 lineal centimetre. In any place where the surface 

 is concave the effect of the surface tension is to 

 suck outwards that is to say, in mathematical lan- 

 guage, to exert negative pressure inwards. Now, 

 suppose in an instant the rigidity to be annulled, 

 and the piece of glass which you see, still undis- 

 turbed by gravity, to become water. The instan- 

 taneous effect of these unequal pressures over its 

 surface will be to set it in motion. If it were a 

 perfect fluid it would go on vibrating for ever 

 with wildly-irregular vibrations, starting from so 

 rude an initial shape as this which I hold in my 

 hand. Water, as any other liquid, is in reality 

 viscous, and therefore the vibrations will gradually 



1 This sum for brevity I henceforth call simply " the curvature of 

 the surface " at any point. 



