228 DISCOURSE ON THE STUDY 



very great influence in modifying the general results 

 of the action of such attractive and repulsive forces 

 as may be assumed to account for the phenomena 

 they present ; yet the general facts that their parts 

 cohere with a certain energy, and that they resist dis- 

 placement or intrusion on the part of other bodies, 

 are sufficient to demonstrate at least the existence 

 of such forces, whatever obscurity may subsist as to 

 their mode of action. 



(243.) This division of bodies into airs, liquids, 

 and solids, gives rise, then, to three distinct branches 

 of mechanical science, in each of which the general 

 principles of equilibrium and motion have their pe- 

 culiar mode of application; viz. pneumatics, hydro- 

 statics, and what might, without impropriety, be 

 termed stereostatics. 



Pneumatics. 



(244.) Pneumatics relates to the equilibrium or 

 movements of aerial fluids under all circumstances 

 of pressure, density, and elasticity. The weight of 

 the air, and its pressure on all the bodies on the 

 earth's surface, were quite unknown to the ancients, 

 and only first perceived by Galileo, on the occasion 

 of a sucking-pump refusing to draw water above a 

 certain height. Before his time it had always been 

 supposed that water rose by suction in a pipe, in 

 consequence of a certain natural abhorrence of a 

 vacuum or empty space, which obliged the water to 

 enter by way of supplying the place of the air sucked 

 out. But if any such abhorrence existed, and had 

 the force of an acting cause, which could urge water 

 a single foot into a pipe, there is no reason why the 



