HOW MAPS ARE MADE 421 



inary lines ate carefully recorded, as we shall see later on, so that they 

 can be accurately laid down at any moment by those who know how to 

 find them. We find them a great convenience, an absolute necessity, 

 indeed, so we leave them drawn on the globe. Imagine a street — any 

 street will do, but for a good analogy imagine a street built, like Moray 

 Place, in a circle. We can say, speaking of, say, a water plug, or any 

 point in that street, that it is on the center line of the street, or the 

 line of the lamp-posts, or so many feet to one side or either of these 

 lines. There is no visibly marked center line or line of lamp-posts, but 

 it can be filled up in a moment by human intelligence, and if there were 

 to be frequent references to them these lines would be marked on apian 

 for constant use. The parallels of latitude are similar lines drawn for 

 convenience or reference. 



The circumference of the earth, like any other circle, is divisible into 

 360 degrees, and we number the parallels by the number of degrees of 

 angular divergence; only, instead of beginning at a pole and going 

 right round, we, for convenience sake, begin at the equator and then 

 number 90 degrees towards the North Pole and 00 degrees towards the 

 South Pole, 



But one set of reference lines is not enough; we nuist have another 

 set, and we get them in the meridians of longitude. We draw these 

 through the poles at right angles to the equator. They are all " great 

 circles;" that is, each circle is concentric with the globe. The equator 

 being a circle, we divide it as before into o60 degrees, and the meridians 

 through the i)oints of section from a second system of lines of reference. 

 But, unlike the iiarallels of latitude, they are all the same size. One is 

 the same as another. How are they to be numbered? Go back for a 

 moment to Moray Place, and remember the Uuca we drew — the center 

 of the street, and the line of the lamp-posts. How are we to define a 

 spot on one of these lines? They are circular, and consequently have 

 no beginning and no end. What we should do would be to mark a con- 

 venient spot with a fiag, or a peg, or a stone, and say "That is the 

 beginning; measure from that.'' 



This is exactly what we must do in longitude. We must mark a 

 starting line on the earth, and call it zero; and as all nations have a 

 free choice they have not chosen the same. We have chosen tlie meri- 

 dian of Greenwich, the French that of Paris, the Americans Washing- 

 ton, and the Eussians Pulkova and the Germans used to use Ferro; 

 but for all English maps, and now for most foreign ones, the meridian 

 of Greenwich is the starting line, the zero of longitude. 



The custom here again is not to reckon 360 degrees round the circle, 

 but to reckon ISO degrees east and 180 degrees west. We saw that 

 latitude was angular divergence from the equator; but what are these 

 degrees of longitude? Look at a ball spinning round opposite a candle. 

 We assumed a knowledge that the earth spun round its axis in twenty- 

 four hours. Every part of it comes in turn opposite a heavenly body 



