68 HISTORY OF MECHANICS. 



cannot say that nature abhors a vacuum at the 

 foot of a mountain more than on its summit."- 

 M. Perrier, Pascal's correspondent, made the obser- 

 vation as he had desired, and found a difference of 

 three inches of mercury, "which," he says, " ravished 

 us with admiration and astonishment." 



When the least obvious case of the operation of 

 the pressure and weight of fluids had thus been 

 made out, there were no further difficulties in the 

 progress of the theory of hydrostatics. When 

 mathematicians began to consider more general 

 cases than those of the action of gravity, there 

 arose differences in the way of stating the appro- 

 priate principles : but none of these differences 

 imply any different conception of the fundamental 

 nature of fluid equilibrium. 



Sect. 2. Discovery of the Laws of Motion of 

 Fluids. 



THE art of conducting water in pipes, and of direct- 

 ing its motion for various purposes, is veuy old. 

 When treated systematically, it has been termed 

 Hydraulics : but Hydrodynamics is the general 

 name of the science of the laws of the motions of 

 fluids, under those or other circumstances. The 

 Art is as old as the commencement of civilization : 

 the Science does not ascend higher than the time 

 of Newton, though attempts on such subjects were 

 made by Galileo and his scholars. 



