ON SHIP WAVES. 455 



shape is proportionate to the speed of the 

 change : the quicker you change the shape, the 

 greater is the force that is required to make 

 the change. Only give it time, and treacle 

 or oil will settle to its level in a glass or basin 

 just as water does. No deviation from perfect 

 fluidity, if the question of time does not enter, 

 has ever been discovered in any of these fluids. 

 In the case of all ordinary liquids, anything that 

 looks like liquid and is transparent or clear 

 or, even if it is not transparent, anything that 

 is common^ called a fluid or liquid is per- 

 fectly liquid in the sense of exerting no per- 

 manent resistance to change of shape. The 

 difference between water and a viscous substance, 

 like treacle or oil, is defined merely by taking 

 into account time. Now for some motions of 

 water (as capillary waves), resistance to change 

 of shape, or as we call it viscosity, has a very 

 notable effect ; for other cases viscosity has no 

 sensible effect. I may tell you this I cannot 

 now prove it, for my function this evening is 

 only to explain and bring before you generally 



