422 HOW MAPS ARE MADE. 



(say the sun) once in twenty-four hours, just as every part of this ball 

 comes opposite the candle once in each revolutiou. Longitude is, tlien, 

 angular divergence measured by the difference of time in coming oppo- 

 site a heavenly body. As the circle is divisible into .300 degrees, so the 

 day,?, e., the revolution of the earth, is divisibkMuto twenty-four hours, 

 and one hour of longitude is consequently equal to 15 degrees. In 

 maps longitude is marked in degrees, while in almanacs the elements 

 given to reckon it are always written in hours, minutes, and secoiuls. 



Remember once more, that these degrees are not. lengths measured 

 on the surface, but are the record of angular divergence from the initial 

 meridian. It is all the more necessary to bear this in mind, because 

 the length on the surface of the earth of a degree of longitude varies 

 enormously, being greatest at the equator and nothing at all at the 

 pole, dift'ering thus from degrees of latitude, which, roughly speaking 

 and for the purposes of this paper, may be considered equal. 



The idea that latitude and longitude are the measures of angular 

 divergence and not absolute distance in miles or yards may be easily 

 grasped by familiar illustration. If you can imagine two travellers leav- 

 ing Italy by road, the one over the St. (iothard pass and the other 

 over Mount Cenis, and two other travellers following them by rail at 

 such an interval of time that they are in the railway tunnels at exactly 

 the same moment that the pedestrians attain the summits of the 

 passes; the pedestrians on the mountain and the railway traveller in 

 the tunnel of the St. Gothard pass will be in exactly the same latitude 

 and longitude, and so will the travellers by the Mount Cenis routes. 

 The pedestrians however will be about 00 yards fiirther apart from 

 each other than the railway travellers. The reason of this of course is 

 that the pedestrians are forther away from the earth's center, but their 

 angular divergence from the ecjuator and the earth's axis are precisely 

 the same, whether they are on the mountain or in the tunnel 5,000 

 feet below. 



Now, having defined latitude and h)ngitudeand shown how the lines 

 representing them are drawn, we must see how in practice the surveyor 

 finds the latitude and longitude of a place, and thereby begins his map. 

 The poles, as we saw, are first points to measure from, and the eijuator 

 the half-way line. It is evident he can not measure directly a line from 

 pole to pole, find out the half and call it the equator, and leave pegs at 

 each i)arallel in passing. He nuist look to things outside the earth 

 itself from which to reckon, and he gets such reference points in the 

 heavenly bodies. To his eye these are situated in the great vault of the 

 heavens. He sees them as if on the surface of -a hollow globe continu- 

 ally revolving around him, rising in the east till they reach their high- 

 est point above him, called the culminating ])oint, then setting in the 

 west. For thousands of years astronomers have studied these bodies, 

 and fixed their api)arent i)Ositions in the celestial vault; and these 

 positions are recorded with the utmost i)ossible accuracy in a book, 



