1 2 Preliminary Remarks and Definitiojis. 



From the very nature of matter as thus presented to our con- 

 sideration, it will be seen that a body once put into a state of mo- 

 tion or rest, by any cause whatever, must continue in that state forev- 

 er, if no new cause is made to act upon it* If it cannot give itself 

 motion when at rest, it cannot stop itself when in motion, for this 

 would be equivalent to giving itself motion in the opposite direc- 

 tion; neither can it change its velocity or direction, for this 

 would equally imply a new force. -Thus, motion is naturally 

 equal or uniform and rectilinear. 



1 8. When several forces are applied at the same time to a 

 body, they are mutually modified by the connexion which exists 

 among the different parts of the body, and which prevents each 

 from taking the motion which the force exerted upon it tends to 

 produce. If these forces happen entirely to destroy each other, 

 so that the body remains at rest, we say that the forces are in 

 equilibrium, or that the body is in equilibrium, under the actioa 

 of these forces. 



19. Mechanics is the science which treats of the equilibrium 

 and motion of bodies. That part, the object of which is to dis- 

 cover the conditions of equilibrium, is called statics.* We give the 

 name of dynamics^ to the other part which has for its object to 

 determine the motion which a body takes, when the forces ap- 

 plied to it are not in equilibrium. The general laws of statics 

 and dynamics are applicable to fluids ; but on account of the 

 peculiar difficulty attending the consideration of this class of 

 bodies we are accustomed to treat them separately. That part 

 of the mechanics of fluids which relates to their eqilibrium is 

 called hydrostatics, J and that which comprehends their motions, 

 hydrodynamics.^ 



20. In our inquiries on these subjects, we first proceed upon 

 the supposition that there are no other bodies, and no other forces, 

 in nature, except those under consideration. Thus all bodies 

 are supposed to be destitute of weight, and free from friction, 

 resistance, and obstructions of every kind. Regard is afterwards 

 had to these causes ; but to estimate their effects, it is necessary 

 to begin by investigating each point separately. 



* From trryfii, I stand. jFrom Jyvse^/s, power 



I" From vap, water, and irmpi. From vfvo and 



