72 POPULAR LECTURES AND ADDRESSES. 



(6.) The latitude of a place is the altitude there 

 of the celestial pole. 



(7.) The longitude of a place is the angle between 

 its meridian and that of Greenwich. 



(8.) In the preceding definitions the term sky is 

 used so as strictly to mean a spherical surface of 

 infinitely large radius, having its centre at the centre 

 of the earth, or at the eye of an observer situated 

 anywhere at the surface of the earth. The great- 

 ness of the radius makes it a matter of no moment 

 whether the centre be at the earth's centre or at 

 the eye of the observer. 



44. (9.) Horizon, derived from a Greek word, 

 which signifies bounding, is the boundary between 

 sky and earth, or sky and sea, as seen by any 

 observer. The term is not usually applied where 

 the sky is cut off by high hills or mountains, but 

 it is usually and properly enough applied to the 

 boundary between earth and sky, as seen by an 

 observer looking over a wide extent of level 

 country from any elevation, great or small. The 

 most common application of the term is to the 

 sea horizon, as described in 5 above. Some- 



