330 PHYSIOGRAPHY. [cuap. 



lines are not parallel, inasmuch as they all meet together and 

 cross at each of the poles. It is common, however, to refer 

 to these imaginary north-and-south circles as meridians, for 

 the reason pointed out on p. 7. 



AVhile latitude is always measured from the equator, 

 longitude has no natural starting-line. The reckoning may 

 begin indeed from any meridian, and different countries 

 actually use different hnes for this purpose. The meridian 

 from which the reckoning begins is called the first, or prime 

 meridian; and, in this country, it is the meridian which passes 

 through the observatory at Greenwich. Greenwich, there- 

 fore, has no longitude ; and, in like manner, all places due 

 north and south of Greenwich have no longitude, since they 

 are on the same meridian. But all places to the east, or to 

 the west, of this first meridian have their longitude, which 

 is expressed in so many degrees or minutes or seconds, and 

 is designated as east or as west, according to its position 

 with reference to Greenwich. Thus London Bridge has a 

 longitude of 5' 10" W. As the equator is divided into 360 

 degrees, it may be supposed that a meridian passes through 

 each of the 360 divisions. Hence a degree of longitude 

 measured at the equator, is the ^y^th part of the circum- 

 ference of the earth. But in going to the north, or to the 

 south, of the equator, the meridians draw closer and closer 

 together until they meet at the poles, as shown in Fig. 105. 

 Each parallel of latitude, whether large or small, is divided, 

 like the equator, into 360° j and, therefore, the length of 

 a degree of longitude gets less and less in passing from the 

 equator, where it measures 60 geographical miles, to either 

 of the poles, where it vanishes altogether. The reckoning of 

 longitude proceeds from the first meridian to the east and 

 to the west, until the figures reach 180°; the reckoning of 

 latitude proceeds from the equator to the north and to the 



