LESSONS IN PHYSICS. 19 



motion. Inertia is the property of matter which does not allow 

 a body to change its condition of rest or motion. 



Leaves and fruit fall to the earth ; force is necessary to lift a 

 body from the earth ; in short, all our experience seems to indi- 

 t-ate the existence of a force which draws or tends to draw all 

 things together into one mass. This force is called attraction. 

 It is known by different names from its different modes of action. 



Gravitation is that form of attraction which acts on all 

 bodies and throughout all distances. Every body or particle of 

 matter in the universe attracts every other body or particle of 

 matter. The rain, the snow, the dead branch, or loosened rock, 

 falls under the action of gravitation; and it also holds the heav- 

 enly bodies in their accustomed orbits. The action of gravity, as 

 manifested on the earth, is to bring all bodies as near the center 

 of the earth as possible and hold them there. 



The force of gravity is found to vary in accordance with two 

 laws. 



If two bodies, one containing twice as much matter as the 

 other, are attracting a third body, equally distant from the 

 others, the force exerted by the larger one will be twice as great 

 as that exerted by the other. If one contained three times as 

 much matter as the other it would exert three times as muchforce, 

 etc., illustrating the first law of gravity, that the force of gravi- 

 tation varies directly as the quantity of matter exerting it; that 

 is, the greater the quantity of matter the greater the force of 

 attraction. 



Suppose a body to be twice as far from a source of attraction 

 at one time as at another. In the first position the force of 

 attraction will be only one-fourth as strong as in the second. 

 If the distance be three times as great the attraction will be 

 one-ninth as strong, etc., illustrating the second law of gravity, 

 that the force of gravitation varies inversely as the square 

 of the distance through which it acts. In discussing gravity, 

 the center of gravity is considered as the center of the mass 

 of matter exerting the force, and the distances are estimated 

 from the center of gravity in each case. In the case of a sphere 



