XIX.] THE FIGURE OF THE EARTH. 327 



is desired to fix the position of the point P, in Fig. 103 ; let 

 any two straight lines be drawn at right angles to each other, 

 such as OA and OB, and then measure how far the point 

 P is from one of them, say OB. Let P be three inches 

 from OB, then it is known that the point must be some- 

 where in the course of the dotted line CD, which is supposed 

 to be three inches from OB. Some clue has thus been 

 obtained to the position of the point, but it is not yet 

 definitely determined. To fix the point, it is necessary to 

 measure its distance also from the other line OA ; let this 

 distance be two inches : it is clear that the position of the 

 point must be somewhere in the 

 line EF, which is everywhere two 

 inches from OA. But as it has ^ 

 also been shown to be in the line 

 CD, it is evident that its position - 

 is fixed at P, for this is the only 

 point at which the two lines cross. 

 The distances three and two, re- ^ ^"j -'^ 



ferred to these lines OB, OA Fig. 103. -Co-ordinates of a point. 



respectively, will accurately mark 



the position of P, and they are called by mathematicians 



the co-ordinates of the point. 



Geographers use co-ordinates in this way to indicate the 

 position of places upon the surface of the earth. When 

 they wish to mark the place of any point, they refer it to 

 certain fixed lines which they imagine to be drawn upon 

 the surface of the globe. They proceed on the convenient 

 fiction that a line is traced entirely round the earth, midway 

 between the two poles ; and this line, which is practically 

 a circle, they call the equator'^ (Fig. 104). The equator 

 consequently divides the world into two equal halves — a 

 ■^ Equator, from Lat. aquo, to make equal. 



-^P. 



