Merid- 

 ians. 



CHAP. XIII.] OBTAINING LONGITUDE 293 



of longitude," or " meridian distance," between tAvo places, 

 neither of which may have its meridian distance from the 

 primary meridian of Greenwich determined ; but by the 

 accumulation of such observations, the absolute longitudes of 

 certain places are from time to time decided. 



These, then, become secondary meridians, on which the Secondary 

 longitude of places in their vicinity depend. 



When, therefore, a secondary meridian is changed in its 

 value as regards its distance from the first meridian, all places 

 whose longitude has been measured from it are changed 

 also. 



This is the work of the Hydrographic Office, which receives 

 and collates all information. The nautical sm'veyor simply 

 finds the difference of longitude, and transmits that informa- 

 tion only. 



A list of secondary meridians is given in the Instructions 

 for Hydrographic Surveyors. 



For our purposes we may look upon difference of longitude Cases 

 as divided into two main cases. The first, where the scale of '^®^®^- 

 a chart we are making depends on the astronomical observa- 

 tions for latitude and longitude at either extremity of our 

 piece of coast. The second, when we wish to determine the 

 relative positions of places more or less far apart, wliich are 

 mainly required for the purposes of navigation. 



In the first, we can nearly always use the system, hereafter 

 described, of " travelling rates," which much adds to the 

 accuracy of the result. 



In the second, time, distance, and general circumstances 

 often prevent our obtaining these, and compel us to use what 

 we can get. 



In obtaining the meridian distance betAveen two places, Principle 

 either by means of a telegraph, or by carrying chronometers °^^^^^^- 

 between them, the principle is the same, and is this — viz., Longi- 

 that the difference of mean time of place at any moment at *"^®' 

 the two places is their difference of longitude in time. 



If, therefore, we can find out that at the time that at a 

 position A it is 9 o'clock, it is 8 o'clock at B, we know that 

 the difference of longitude is equal to one hour of time, and 

 that A is east of B. The telegraph enables us to do tliis in 

 its simplest form, as, ascertaining the exact time at each end 



