346 HISTORY OF MECHANICS. 



of ebony sinks in water, while aflat slip of the same material lies on 

 the surface ; and it required considerable sagacity to separate such 

 cases from the general rule. Galileo's opinions were attacked by 

 various writers, as Nozzolini, Vincenzio di Grazia, Ludovico delle Co- 

 lombe ; and defended by his pupil Castelli, who published a reply in 

 1615. These opinions were 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, inclosed 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 Mechanics, 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 equi- 

 librium of other machines. This, indeed, Galileo had done before him. 

 It followed from this doctrine, that the pressure 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 doctrines 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 diffusing itself when con- 

 densed, as the causes of wind ; but we can hardly consider such pro- 

 priety of phraseology in him as more than a chance ; for we see the 

 value of his philosophy by what he immediately adds : " Do you think 

 that we have forces by which we move ourselves, and that the air is 

 left without any power of moving? when even water has a motion of 

 its own, as we see in the growth of plants." "We can hardlv attach 



O 1 " 



much value to such a recognition of the gravity and elasticity of 

 the air. 



Yet the effects of these causes were so numerous and obvious, that 

 the Aristotelians had been obliged to invent a principle to account for 

 them; namely, "Nature's Horror of a Vacuum." To this principle 

 were referred many familiar phenomena, as suction, breathing, the 



1 (Jti<xst. Sat. \. 5. 



