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 vacuum is a por- 
tion of space destitute of matter. 
By 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 weighty 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, 
weighs 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; 
Dsr 
