388 Hydrodynamics. 



Of the Resistance of Fluids to Bodies moving in them. 



500. THE force with which solid bodies moving in fluids, as 

 water, air, &c., are impeded and retarded, is usually termed the 

 resistance of fluids ; and as all our machines move either in water 

 or in air, or both, it becomes a matter of importa ce in the 

 theory of mechanics to inquire into the nature of this kind of 

 force. 



We know by experience that force must be applied to a body 

 in order that it may move through a fluid, such as air or water ; 

 and that a body projected with any velocity is gradually retard- 

 ed in its motion, and generally brought to rest. We also know 

 that a fluid in motion will hurry a solid body along with it, and 

 that force is necessary to maintain the body in its place. And 

 as our knowledge of nature teaches us that the mutual actions 

 of bodies are in every case equal and opposite, and that the ob- 

 served change of motion is the only indication and measure of 

 the changing force, we infer that the force which is necessary 

 to keep a body immovable in a stream of water, flowing 

 with a certain velocity, is the same with that which is requir- 

 ed to move this body w r ith an equal velocity through stagnant 

 water. 



A body in motion appears to be resisted by a stagnant fluid, 

 because it is a law of mechanical nature that force must be 

 employed in order to put any body in motion. Now, the body 

 cannot move forward without putting the contiguous fluid in 

 motion, and force is to be used to produce this motion. In 

 like manner, a quiescent body is impelled by a stream of 

 fluid, because the motion of the contiguous fluid is diminished by 

 this solid obstacle ; the resistance, therefore, or impulse, differs 

 in no respect from the ordinary communication of motion among 

 solid bodies, at least in its nature ; although it may be far more 

 difficult to reduce the various circumstances attending it to accu- 

 rate computation, or to obtain all the requisite data on which to 

 found the calculation. 



