INTRODUCTION. 11 



one minute; if 60 of those miles, it is a degree, 

 &c. 



. A plane at any place perpendicular to the line in 

 which bodies gravitate, is called a horizontal plane ; 

 and any plane passing through that line is called a 

 vertical plane. 



25. If we conceive the weight of bodies to be 

 produced by a force applied to each of their ele- 

 mentary particles, urging it downwards, this force is 

 palled gravity. 



a. Gravity must be distinguished from Weight : the 

 weight of a body is the product of the gravity of a 

 single particle by the number of particles. Thus, 

 if W be the weight of a body, g the gravity of 

 a single particle, N the number of particles, 



26. In all bodies the force of gravity is the 

 same. 



. This is proved by the fact, that all bodies, what- 



ever be their weight, fall at the same rate to the 



ground, when the resistance of the air is remo- 

 ved. 



27. The weights of two bodies are to one another 

 as the quantities of matter in those bodies, 



a. Let 



