HOW MAPS ARE MADE. 423 



compiled by Government, ealled the N((ntu'«l Almanac, and, from the 

 practical information given there, the surveyor finds his position. He 

 may take tlie sun or he nuiy take the stars, but the jiositions of the 

 sun being affected by tlie motion of the earth round it, I proi)ose to 

 take a star to illustrate my next rennirks, as its movements are simpler. 



The pole of the heavens is tlie end of the axis of the earth infinitely 

 prolonged. The intersection of the plane of the equator with the celes- 

 tial vault is called the equinoctial, and as the angular divergence on 

 the surface of the earth is measured in degrees from the eipiator and 

 called latitude, so the angular divergence of a heavenly body from the 

 equinoctial is called its <lecliiiatio)i. As the angular divergence from 

 the meridian of Greenwich was called longitude, so the divergence in 

 time from a starting i)oint in the heavens is called right ascension. We 

 had to fix arbitrarily the meridian of Greenwich as a starting" line on 

 the earth. We have also to fix equally arbitrarily a starting i)oint in 

 the heavens, and that i^oint may be most simply described as the 

 point In the heavens in which the sun is in spring, when the day and 

 night are equal. 



The latitude of any place on the earth's crust is equal to the altitude 

 of the celestial pole. You can see this in a moment if you imagine 

 yourself on the equator and look to the pole, marked, say, by the Pole 

 Star. You will see it on the horizon and of no altitude at all; and at 

 the equator you have no lat tude, or it is called zero; but, as you 

 approach the pole, the Pole Star will gradually appear to rise higher 

 and higher until when you reach the ISTorth Pole it will be directly over 

 your head, and consequently at right angles to, or 90 degrees from the 

 horizon, and your latitude is then also 90 degrees. But thougli the 

 Pole Star is very near the Xorth Pole, it does not actualy coincide 

 with it, and we must find some other way of finding our latitude accu- 

 rately. We get this by taking the altitude of any known star in vari- 

 ous ways. I Avill explain the sinqjlest method, of which all others are 

 only slight modifications : 



(1) Measure the meridian altitude of the star — that is, its highest alti- 

 tude above the horizon. 



(2) Deduct that altitude from 90 degrees, which gives its zetiith dis- 

 tance, or the angular distance from a j)oint exactly over your head. 



(3) Add (or substract) the declination of the star (found in the Nau- 

 tical Almanac) to the zenith distance, and the result is your latitude. 



I have here a diagram showing how the latitude of Edinburgh would 

 be found from the bright star Arcturus, which " culminates," or reaches 

 its highest altitude, on our meridian a few minutes before 12 to-night. 



1 measure first its altitude, which 1 find is 54 degrees. Deducting 

 that from 90 degress gives its zenith distance = 36 degrees; to that I 

 add its declination, which I find from the almanac is 20 degrees, and 

 the result is 50 degrees ^ the latitude of Edinburgh. 



Longitude is a more difficult matter, and 1 have no time to go into 

 it anything like fully. You will find a beautiful description of it in 



