40 MECHANICAL POWERS. 



of the same length vibrate in the same time however differ* 

 ent in weight. 



Questions. — 1. In what direction will a body move when impel- 

 led by two forces ? 2. Describe the motion of a ship as impelled by 

 the wind and a current. 3. What is circular motion? 4. The ex- 

 ample ? 5. Centripetal force ? 6. Centrifugal ? 7. What is said of 

 these two forces ? 8. What is a tangent ? 9. What is said of the 

 motion of the moon? 10. What is a parabola ? 11. A pendulum? 

 12. Describe the manner in which a pendulum .vibrates. 13. Why ia 

 not the motion of a pendulum perj)etual ? 14. Why do clocks go 

 faster in winter than in summer ? 15. Why do pendulums vibrate 

 faster towards the poles than at the equator ? 



Note. Tlie centrifugal force is stronger at the equator than at the 

 poles J and as it tends to drive bodies from the centre, it is necessarily 

 opposed to, and must lessen the power of gravity, which attracts them 

 towards the centre. The equatorial diameter of the earth is stated 

 by some to be 34 miles, and by others to be 26 miles longer than the 

 polar diameter. IG. Illustrate by figure 1. the composition and re- 

 solution of motion. 



LESSON 20. 



Mechanical Powers. 



Centre of motion is that point which remains at rest wliile all 



the other parts of a body move round it. 

 Axis of motion is the line about which a revolving body moves. 

 Equilib'rium, equipoise, equality of weight. 



The mechanical powers are simple instruments or ma- 

 chines in the hands of man, by which he is enabled to raise 

 great weights, and overcome such resistances as his natural 

 strength could never effect without them. They are six in 

 number, the lever, the pulley, the wheel and axle, the in- 

 clined plane, the wedge, and the screw, one or more of which 

 enters into the composition of every machine. In order to 

 understand the power of a machine, four things are to be 

 considered ; the power that acts, which consists in the effort 

 of men or horses, of weights, springs, running waters, wind, 

 and steam ; the resistance which is to be overcome by the 

 power, which is generally a weight to be moved ; the centre 

 of motion, or, as it is termed in mechanics, the fulcrum, which 

 is the point about which all the parts of a body move ; and 

 lastly, the respective velocities of the power, and of the re- 

 sistance, which must depend upon their respective distances 



