10 



PHYSICAL PROPERTIES. 



Extension is the property of occupying a certain 

 portion of space. This is called the volume of a 

 body, and the quantity of matter which it contains 

 is its mass. Every substance in nature has length, 

 breadth, and thickness ; and the limits of its exten- 

 sion are called figure or shape. A vacuufn is a por- 

 tion of space destitute of matter. 



By the gravity of a body is meant its tendency to 

 fall towards the earth ; and the power by which it 

 is thus drawn is called the attraction of gravitation. 

 The force required to separate a body from the sur- 

 face of the earth is called its weight, and this is in 

 proportion to its quantity of matter. Every par- 

 ticle of matter in the universe is under the influence 

 of this principle ; by it the moon is drawn towards 

 the earth ; while both these, together with all the 

 other planetary bodies, are drawn towards the sun. 

 Solar systems, of which each fixed star, so called, 

 constitutes the centre, are thus bound together; 

 while all of them, there is good reason to believe, 

 revolve around some common centre in the bounds 

 of infinite space. 



The velocity of a falling body is accelerated in 

 proportion to the height of its fall, which proves 

 that the force of gravity is greatest at the surface 

 of the earth, from whence it decreases, both up- 

 ward and downward ; so that, at the centre of the 

 earth, a body would be entirely at rest. The ratio 

 of this diminution is as the square of the distance, 

 the square of any number being that number mul- 

 tiplied into itself. Thus, if a body on the surface of 

 the earth, which is 4000 miles from its centre, 

 w^eighs one pound, if it be carried twice this dis- 

 tance from the centre, or 8000 miles, it will weigh 

 but a quarter of a pound ; and if carried three times 

 the distance from the centre, it would weigh but 

 one ninth of a pound, or less than two ounces. 

 Now the moon is 60 times as far from the centre 

 of the earth as the centre is from the surface ; 



