64 HISTORY OF MECHANICS. 



generally adopted and diffused; but somewhat later, 

 Pascal pursued the subject more systematically, and 

 wrote his Treatise of the Equilibrium of Fluids in 

 1653; in which he shows that a fluid, enclosed in a 

 vessel, necessarily presses equally in all directions, 

 by imagining two pistons, or sliding plugs, applied 

 at different parts, the surface of one being centuple 

 that of the other : it is clear, as he observes, that 

 the force of one man acting at the first piston, will 

 balance the force of one hundred men acting at the 

 other. "And thus," says he, "it appears that a 

 vessel full of water is a new Principle of Mecha- 

 nics, and a new Machine which will multiply force 

 to any degree we choose." Pascal also referred 

 the equilibrium of fluids to the " principle of virtual 

 velocities," which regulates the equilibrium of other 

 machines. This, indeed, Galileo had done before 

 him. It followed from this doctrine, that the pres- 

 sure which is exercised by the lower parts of a fluid 

 arises from the weight of the upper parts. 



In all this there was nothing which was not 

 easily assented to : but the extension of these doc- 

 trines to the air required an additional effort of 

 mechanical conception. The pressure of the air on 

 all sides of us, and its weight above us, were two 

 truths which had never yet been apprehended with 

 any kind of clearness. Seneca, indeed 1 , talks of 

 the "gravity of the air," and of its power of dif- 

 fusing itself when condensed, as the causes of wind : 



i, Nat. v. 5. 



