INTRODUCTION. 17 



The science of dynamics, thus modified, is called 



MECHANICS. 



. According to this distinction, the motion of a body 

 falling freely to the ground belongs to Dynamics ; 

 the motion of the same body descending on an in- 

 clined plane, belongs to Mechanics, &c. 



t. The doctrine of Machines belongs to mechanics; 

 for in every machine, the connection of the parts 

 prevents them from immediately obeying the im- 

 pulses which they receive. 



40. When the bodies to which motion is commu- 

 nicated are fluid, another modification of the prin- 

 ciples of dynamics takes place, which constitutes 

 the science of HYDRODYNAMICS. 



In each of the three sciences just named, there are 

 cases where the forces balance one another, and pro- 

 duce, not motion, but rest. These cases in dyna- 

 mics and mechanics constitute a particular branch, 

 to which the name of Statics has been given. The 

 similar cases in fluids form the branch of hydrodyna- 

 mics, called Hydrostatics. 



Hydrodynamics is also subdivided, according as the 

 fluids are incompressible, like water, or elastic, like 



41. ASTRONOMY is .the science which describes 

 the phenomena of the heavenly bodies ; and PHY- 

 SICAL ASTRONOMY is the part of that science which 



VOL, L B applies 



