Lesson XIII. 1 



N VI IKAL PHILOSOPHY. 



29 



T. What is the use of a pendulum 

 to a c! 



/'.On account of its uniform vibration 

 otion, for without it the 

 wheels would go very irregularly. 



T. As pendulums are required 

 - ite only sixty times in a minute, 

 . neither slower or quicker ; how is 

 Managed ? 

 P. By adjustment of the length. 



GENERAL QUESTIONS ON LESSON XII. 



1. How many kinds of equilibrium are 

 there, and how are they distinguished ? 



2. What is a pendulum .' 



3. "Wh.it causes its oscillation? 



4. Why does the sweep of a pendulum, 

 left to itself, decrease in length each time ? 



hy does the length of the rod affect 

 the vibrations of the pendulum ? 





LESSON XIII. 



RECAPITULATION, &c. IT is quite as natural for a body to be in a state of motion 



as rest, according to the laws of Natural Philosophy, because it has been shown thnt 



is passive, and must therefore be influenced by some external force to make it 



change its condition from a state of rest to that of motion, or vice versa. Sir Isaac 



i laid down three propositions, which have been called the " laws of motion." 



They are as follow : 



1st " Every body must persevere in its state of rest, or of uniform motion in a 

 straight line, unless it be compelled to change that state by forces impressed upon it." 



2nd. " Every change of motion must be proportional to the impressed force, and 

 mutt be in the direction of that straight line in which the force is impressed." 



3rd. " Action must always be equal to and contrary to reaction ; or the actions of 

 two bodies upon each other must be equal, and directed towards contrary sides." 



.1 he our duty to consider these laws, and examine the phenomena they 

 present We have already found that bodies cannot possibly continue in a state 

 of motion for any length of time upon or near the earth ; for the attraction of 

 gravitation, atmospheric resistance, and friction, would operate so powerfully, that the 

 bodies would be arrested in their course. It is therefore evident, that permanency or 

 uniformity of motion can only be fully demonstrated by the heavenly bodies, which 

 have continued to move, with uniform force, since the time the Creator launched them 

 into space. 



QUI 



164. r. Suppose that there are two 

 equal forces acting upon one point from 

 oppOM .-, what is the result .' 



l>ody acted upon will 

 in a state- of equilibrium. 



two forces act in the 

 same direction, what in 



P. i ill he equal 



to that of a double load, and so on for 

 every additi< 



ami tl,. iev can only 



produce one motion in ouc definite dircc- 



n ply that a single 

 force would produce the same cil< 



-this single force called? 

 /'. The rctultant or rywjeW, <.' 

 * \.r.nj ! . \vlicn a ship is impelled by tho 



<d action of the stream. rml<i 

 wind, it will move in a d 

 hut if the actions of those force* v 

 cease, the ship would he in.-. 



;. loving n 

 :<-h as a rope, which 



rcction a* 

 -1 hy the three forces ; const'; 



