Lesson XIV.] 



NA'ITRAL PHILOSOPHY. 





in a vertical plane, is attached. Let the ver- 

 tical planes of both pulleys coincide, then 

 pass a line having a weight (a) at one 

 end, and another weight (c) at the other ; 

 now suspend a weight (b) at any point 



(o) between the pulleys, the whole will be 

 in a state of equilibrium in any definite 

 position of the lines. There arc three 

 forces acting upon the point o, in the 

 directions o p, o q, and o r, and we 

 may ascertain the resultant by construc- 

 tion. 



r. Will vou explain this more 



P. Suppose that the weight a was equal 



ml c. to 3 ounces, the force or 



u'ht at b requisite to produce an 



angle of 76 must be four ounces. Let 



the angle a b c measure 75, and a b= 2, 



and b c = 3 (the length being given), then 

 d the diagonal 



= 4. To 

 prove this, 

 if we attach 

 a four ounce 

 weight 19 

 the line, we 

 shall find 

 that the an- 

 gle p o q 

 measures 

 75. From 

 this it ap- 

 pears that 

 the greater 

 the weight 

 at o the less 

 will be the 



angle p o q, and the less the weight the 



greater the angle. 



GENERAL QUESTIONS ON LESSON XIII. 



1. How do you define force? 



'2. What are the laws of motion laid 

 down by Sir Isaac Newton ? 



3. NY hat is the resultant 7 



4. What is meant by the diagonal of the 



hat is the parallelogram of forces ? 

 6. Prove the correctness of the law of the 

 parallelogram of forces. 



LESSON XIV. 



RECAPITULATION, &c. THE laws of equilibrium in all simple machines are <i 

 from the knowledge of the parallelogram of forces ; for example, the inclined plane, 

 the screw, the wedge, the pulley, and the lever. We have now to consider the motions 

 produced by gravity, where the directions of the force of gravitation in various points 

 longer parallel. Bodies have a tendency to proceed in a straight line when Hying 

 > we may see every day ; and this property is termed the centri- 

 fugal or centre flying force. 



QUESTIONS. 



Vhat is rr.NTKIFirOAL FORCE? 

 /' It in that force which causes all 

 bodies to fly ofTfrom t: 



pupil hould pir. example of 

 fug.l force, tuch a, tbr trundling of A mnii, 

 tht whirling of lirg with a tone in 

 it. I 



r. Is the earth subject to this 

 force ? 



/'. Certainly. Win-never tin re is rota- 



nnd a fixed axi*. And the separate 



'g from 



this axis in any way, this force must 



