XIX] THE FIGURE OF THE EARTH, 329 



The distance from the equator to the north pole is one- 

 fourth of the earth's circumference, and therefore the latitude 

 of the pole is said to be 90°, or one-fourth of 360° measured 

 from the equator northwards. In like manner, the south pole 

 is in 90° south latitude. London is described as being in 

 " 51° 30' N. lat. " ;^ a description which tells us at once that 

 London is situated in the northern hemisphere, at a distance 

 of 5ii°, or about 3,560 statute miles from the equator. The 

 latitude of so large a place as London will of course vary 

 in different parts : the middle of London Bridge is in 5 1° 

 30' 24" N. lat. 



Latitude alone could never fix the position of a place. 

 Any number of places, for example, might be situated, 

 like London, on the circle which runs round the northern 

 hemisphere at 51^° from the equator. Two sets of 

 standard lines are therefore needed, just as two lines were 

 required in Fig. 103. Geographers have consequently been 

 led to draw a number of imaginary circles round the globe, 

 all running through the north and south poles, as in Fig. 105. 

 These are called lines of longitude, and they differ in several 

 respects, beside that of direction, from lines of latitude. All 

 lines of longitude form circles which have the earth's centre 

 as their centre ; in other words, they are all great circles. 

 But all the lines of latitude, except the equator, are small 

 circles. Again, the lines of latitude form equidistant circles, 

 and are hence commonly called parallels of latitude. But 

 no one can speak of " parallels of longitude," because these 



^ Each degree of latitude is divided into 60 equal parts called minutes, 

 and each minute into 60 equal parts called seconds. Degrees are repre- 

 sented by the symbol °, minutes by ', and seconds by ". Minutes and 

 seconds of tivie are distinguished by the initials m and s respectively. 

 A minute of latitude is a geographical mile, called by nautical men a 

 knot. The geographical mile contains 202S yards, while our statute 

 mile contains only 1760 yards. 



