454 POPULAR LECTURES AND ADDRESSES. 



produce or to maintain that procession of waves ? 

 We all know it does take power to drag a boat 

 through a canal ; but we do not always think 

 on what part of the phenomena manifested by 

 the progress of the boat through the canal, the 

 power required to drag the boat depends. 



I shall ask you for a time to think of water 

 not as it is, but as we can conceive a substance 

 to be that is, absolutely fluid. In reality 

 water is not perfectly fluid, because it resists 

 change of shape ; and non-resistance to change 

 of shape is the definition of a perfect fluid. 

 Is water then a fluid at all ? It is a fluid 

 because it permits change of shape; it is a fluid 

 in the same sense that thick oil or treacle is a 

 fluid. Is it only in the same sense ? I say 

 yes. Water is no more fluid in the abstract 

 than is treacle or thick oil. Water, oil, and 

 treacle, all resist change of shape. When we 

 attempt to make the change very rapidly, 

 there is a great resistance; but if we make the 

 change very slowly, there is a small resistance. 

 The resistance of these fluids to change of 



