68 ELEMENTS OF SCIENCE 



they serve to help us on to a better knowledge of the 

 laws which govern fluid bodies. 



As to liquids, we may assume them to be made up 

 of particles, which, instead of cohering stably in some 

 definite order (as we assume to be the case with the par- 

 ticles of solid bodies), have no tendency to preserve any 

 reciprocal positions, but can move and glide over each 

 other with perfect freedom and in all directions, each par- 

 ticle pressing equally on all the particles which surround 

 it and being equally pressed on by them. 



By this hypothesis we may understand the great 

 difference which exists between a liquid and a solid. 

 Hitherto our conception of pressure due to weight, has 

 been simply downwards, through the force of gravity. 

 But a portion of liquid presses equally in all directions, 

 in consequence of the action of gravity, or of any other 

 force acting upon it. 



Therefore for liquid to be in equilibrium, every particle 

 of it must press and be pressed upon equally in all direc- 

 tions. One consequence of this is that the surface of a 

 liquid, apart from any disturbing influence, must be 

 horizontal. An illustration of the imagined condition of 

 liquids, may be obtained by considering the consequences 

 which would be produced should a fresh comer try to 

 effect an entrance into a room already filled by a crowd 

 of persons, The new comer who manages to effect such 

 an entrance will produce pressure in all directions on 

 every side of him. 



From the mobility of its particles it follows that a 

 liquid immediately takes the figure of any vessel in 

 which it may be received. Therefore, if two or more 

 vessels, however different their sizes, which contain liquid 

 of the same kind, be placed in communication (e.g., by 

 turning stop-cocks) below the surface of the liquid in any 



