OF NATURAL PHILOSOPHY. 2i'5 



yield is so excessively small as to be demonstrably 

 incapable, in most cases, of having any influence 

 on the results : and in those where it has such in- 

 fluence, an especial investigation of its amount can 

 always be made. This gives rise to two subdivisions 

 of the application of mechanical reasonings to solid 

 masses. Those which refer to the action of forces 

 on flexible or elastic, and on inflexible or rigid, 

 bodies, comprehending under the latter all such 

 whose resistance to flexure or fracture is so very 

 great as to permit our adoption of the language and 

 ideas of the extreme case without fear of material 

 error. 



(238.) In like manner, when we reason respecting 

 the action of forces on a fluid mass, all we have 

 occasion to assume is, that its parts are freely move- 

 able one among the other. If, besides this, we 

 choose to regard a fluid as incompressible, and 

 deduce conclusions on this supposition, they will 

 hold good only so far as there may be found such 

 fluids in nature. Now, in strictness, there are none 

 such ; but, practically speaking, in the greater number 

 of cases their resistance to compression is so very 

 great that the result of the reasoning so carried on 

 is not sensibly vitiated; and, in the remaining cases, 

 the same general principles enable us to enter on a 

 special enquiry directed to this point : and hence the 

 division of fluids, in mechanical language, into com- 

 pressible and incompressible, the latter being only 

 the extreme or limiting case of the former. 



(239.) As we propose here, however, only to 

 consider what is the actual constitution of nature, 

 we shall regard all bodies, as they really are, more 



