MECHANICAL PRINCIPLES OF FLUIDS. 65 



but we can hardly consider such propriety of 

 phraseology in him as more than a chance ; for we 

 see the value of his philosophy by what he immedi- 

 ately 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 hardly attach 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 Horrour of a Vacuum." To this principle 

 were referred many familiar phenomena, as suction, 

 breathing, the action of a pair of bellows, its draw- 

 ing water if immersed in water, its refusing to open 

 when the vent is stopped up. The action of a cup- 

 ping instrument, in which the air is rarefied by 

 fire; the fact that w^ter is supported when a full 

 inverted bottle is placed in a basin ; or when a full 

 tube, open below and closed above, is similarly 

 placed ; the running out of the water, in this 

 instance, when the top is opened ; the action of a 

 siphon, of a syringe, of a pump; the adhesion of two 

 polished plates, and other facts, were all explained 

 by tliefuga vacui. Indeed, we must contend that 

 the principle was a very good one, inasmuch as it 

 brought together all these facts, which are really of 

 the same kind, and referred them to a common 



VOL. II. F 



