KINETIC OR MECHANICAL VIEW OF NATURE. 59 



mathematically what is meant by a fluid. The chief 

 property of a fluid, as compared with a solid body, is 

 the perfect mobility of its parts, the absence of rigidity. 

 Thus there were two possible kinds of fluids those 

 which retained their bulk or volume, whilst offering no 

 resistance to change of shape, and those which tried to 

 expand, and could be compressed by means of external 

 forces. These latter were called gases. In dealing with 

 the former, incompressibility had to be defined mathe- 

 matically, as also perfect mobility. These properties 

 constitute what is called a perfect fluid. Such perfect 

 fluids do not exist in nature ; but the method of 

 reasoning was to begin with an ideal, simple case, and 

 approach the explanation of natural phenomena by a 

 process of correction, introducing more and more com- 

 plications. The phenomena of the flow of liquids, 

 practically by far the most important, could be studied 

 to a great extent by means of the simplest form of the 

 hydrodynamical conception, and up to the middle of the 

 century such problems, as well as those of the propa- 

 gation of small displacements under the action of external 

 forces, notably the motion of waves, formed the prin- 

 cipal problems which were treated mathematically. The 

 idea of the friction of fluids, also called viscosity, had been 

 excluded in the definition of a fluid, inasmuch as friction 

 opposed the notion of perfect mobility of the parts, which 

 was the mathematical definition of a fluid. Now it is a 

 matter of experience that in all liquids with which we 

 are acquainted friction can produce rotational motion, 

 such as whirls and eddies ; it was also found that other 

 forces, such as magnetic forces, are, under certain con- 



