46 PRACTICAL LESSONS IN SCIENCE. 



tical, and perhaps more interesting than the other. Every ripple 

 and wave, every swaying branch and rustling leaf, each nodding 

 flower, each bird and beast moves in ever varying curves. 



Curved motion is produced by the action of at least two forces, 

 one of which must be a constant force. The motion of a ball, when 

 fastened to a string and whirled around the hand, is an example 

 of curved motion. The string represents a constant force, and 

 the muscles of the arm an impulsive force. An impulsive force, 

 acting alone, or any number of impulsive forces acting together, 

 tend to cause motion in a straight line, but one or more impul- 

 sive forces acting with a constant force, gives motion in a curved 

 line. The forces which cause circular motion are called central 

 forces ; one acts toward the center, from which the curve is de- 

 scribed, while the other acts in the direction of a tangent to the 

 curve. The one is called the centripetal force, and the other the 

 centrifugal force. The grandest examples of the actions of these 

 forces are found in the motions of the heavenly bodies, gravita- 

 tion acting as the centripetal force. 



The centrifugal force varies as the quantity of matter in the 

 moving body ; that is, the more matter there is in the body the 

 greater will be the reaction against the centripetal force. It also 

 varies as the square of the velocity of the body in motion. These 

 laws are illustrated in a general way by the use of a sling ; the 

 heavier the stone the greater the reaction or pull, and the greater 

 the velocity the greater the pull. When a wagon moves slowly 

 along the road no mud is thrown from the wheels, but as the mo- 

 tion is increased the centrifugal force overcomes the force of ad- 

 hesion between the mud and the wheel, and the mud is thrown 

 off. This principle is made use of in the centrifugal machines 

 employed in laundries for drying clothes, in sugar refineries for 

 removing molasses from the sugar, and in apiaries for extract- 

 ing honey from the comb. It accounts for the action of the gov- 

 ernor on a steam engine; for bursting grindstones and balance 

 wheels; and the increased size of the earth at the equator was 

 undoubtedly caused by centrifugal force when the earth was cool- 

 ing from the liquid state. The effect of centrifugal forces may be 



