OF NATURAL PHILOSOPHY. 233 



remarkable for the directness of its application to 

 useful purposes. The immediate and perfect distribu- 

 tion of a pressure applied on any one part, however 

 small, of a fluid surface through the whole mass, en-r 

 ables us to communicate at one instant the same pres- 

 sure to any number of such parts by merely increasing 

 the surface of the fluid, which may be done by 

 enlarging the containing vessel ; and if the vessel 

 be so constructed that a large portion of its surface 

 shall be moveable together, the pressures on all the 

 similar parts of this portion will be united into one 

 consentient force, which may thus be increased to 

 any extent we please. The hydraulic press, in- 

 vented by Bramah, (or rather applied by him after 

 a much more ancient inventor, Stevin,) is con- 

 structed on this principle. A small quantity of 

 water is driven by sufficient pressure into a vessel 

 already full, and provided with a moveable surface 

 or piston of great size. Under such circumstances 

 something must give way ; the great surface of the 

 piston accumulates the pressure on it to such an 

 extent that nothing can resist its violence. Thus, 

 trees are torn up by the roots ; piles extracted from 

 the earth ; woollen and cotton goods compressed 

 into the most portable dimensions ; and even hay, 

 for military service, reduced to such a state of 

 coercion as to be easily packed on board transports. 

 (252.) Liquids differ from aeriform fluids by 

 their cohesion, which may be regarded as a kind 

 of approach to a solid state, and was so regarded 

 by Bacon (193.). Indeed, there can be little doubt 

 that the solid, liquid, and aeriform states of bodies 

 are merely stages in a progress of gradual tran- 



