6 OUTLINES OF NATURAL PHILOSOPHY. 



a. The direction of gravity passes nearly through the 

 centre of the earth. 



b. The Zenith and the Nadir are the Poles of the 

 horizon. 



c. If a plane parallel to the plane of the horizon of 

 any place, pass through the centre of the earth, it 

 will cut the celestial sphere in a circle hardly di- 

 stinguishable from the former. It is called the 

 Hational 9 and the other the Sensible Horizon. 



d. The Horizon of any place divides the visible from 

 the invisible Hemisphere. This supposes the eye 

 to be situated on the surface of the earth ; if it is 

 elevated above the surface, the apparent horizon is 

 a circle parallel to the former, but lower, being the 

 base of a cone which has the eye of the spectator 

 for its vertex, and of which the superficies touches 

 the superficies of the earth all round. 



10. Circles, the planes of which pass through 

 the Zenith and Nadir of any place, are called 

 Vertical Circles, and are perpendicular to the ho- 

 rizon of the place. 



a. The Meridian of any place, is one of the vertical 

 circles of that place. 



b. The vertical circle that is at right angles to the 

 plane of the meridian, is called the Prime Verti- 

 cal. 



If 



