72 OF SIMPLE ACCELERATING FORCES. 



mnnner, the general result of any number of motions may 

 be obtained, by addition aiid subtraction only. Thus, if a 

 bird ascended in an oblique direction, we might describe 

 its flight by estimating its progress northwards or south- 

 wards, eastwards or westwards, and at the same time up- 

 wards, as accurately as if we ascertained the immediate 

 bearing and angular elevation of its path, and its velocity 

 in the direction of its motion. 



SECTION II. OF SIMPLE ACCELERATING FORCES. 



228. Definition. Any immediate cause 



of a change of motion is called a force. 



Scholium 1. The word force ought to be very strictly 

 confined to a cause which produces motion in a body at 

 rest, or which increases, diminishes, or modifies it in a 

 body which was before in motion. Thus, the power of 

 gravitation, which causes a stone to fall to the ground, is 

 called a force ; but when the stone, after descending down 

 a hill, rolls along a horizontal plane, it is no longer im- 

 pelled by any force, and its relative motion continues un- 

 altered, until it is gradually destroyed by the retarding 

 force of friction. It was truly asserted by Descartes, 

 that the state of motion is equally natural with that of rest, 

 and that when a body is once in motion, it requires no 

 foreign power to sustain its velocity. Since, however, the 

 inertia of one body may easily become the cause of motion 

 in another which is impelled by it, the term force is not 

 uncommonly employed as almost synonymous with motion, 

 and hence has arisen the incorrect notion of the vis inertiae, 

 and of the force possessed by a moving body : but we must 

 be careful to recollect that this sense of the term force is 

 only so far correct, as it is applied to the power of causing 



