486 The National Geographic Magazine 



laid down by estimation of the dis- 

 tance traversed and direction of march, 

 checked perhaps by a few astronomical 

 observations of more or less value as the 

 traveler possesses or does not possess the 

 necessary skill. 



The compilers of such a map have a 

 difficult task. Discrepancies are, of 

 course, multitudinous. Nothing agrees, 

 and one has to accept, reject, and ad- 

 just as best he can on his own responsi- 

 bility and with what knowledge he can 

 procure of the respective trustworthi- 

 ness of each author. 



Happy is he if he has even a few po- 

 sitions on his map which have been 

 properly determined, as between them 

 he is saved from the constantly increas- 

 ing errors of adding one little area to 

 another, which if carried on indefinitely 

 culminates in great errors. 



Of course such maps are of no prac- 

 tical use, save as giving a very general 

 idea of a country, and when required 

 by the administrator or traveler lead to 

 endless mistakes and annoyances. 



THE COAST LINE OF THE WORLD 

 PRACTICALLY DETERMINED 



The feature of our globe which is 

 now, broadly speaking, most accurately 

 laid down is the coast line. The safety 

 of navigation has caused general ma- 

 rine surveys to be carried on all over 

 the world during the nineteenth cen- 

 tury, which have finally determined the 

 position and shape of the boundaries of 

 the sea. 



These surveys, executed for the most 

 part by skilled naval officers with proper 

 instrumental outfit, and supplied espe- 

 cially with trustworthy chronometers, 

 and based upon frequent carefully de- 

 termined astronomical positions, have 

 resulted in this boundary line being de- 

 lineated with an accuracy, so far as its 

 absolute position is concerned, far in 

 advance of any other main feature in 

 maps. Here I may perhaps explain to 

 those unversed in these matters why 

 this is so. 



The position of any spot on the earth's 

 surface can be ascertained in two ways : 

 either by careful measurement by means 

 of an accurate system of triangles from 

 another spot already fixed, or by inde- 

 pendent observations of the heavenly 

 bodies and calculations from them, 

 which give the precise latitude and lon- 

 gitude of the place. The former is 

 suitable for positions inland, but en- 

 tails much time and labor, and is only 

 adopted when a perfect map is to be 

 made, for which it is the indispensable 

 foundation. The latter can be carried 

 on from a ship, and in most circum- 

 stances only from a ship, because of the 

 limitations of the methods of determin- 

 ing longitudes. 



Longitude can now be satisfactorily 

 and rapidly ascertained in two ways — 

 by the electric telegraph or by use of 

 chronometers. 



The places served by the electric tele- 

 graph are still few, and its use is there- 

 fore restricted ; but the chronometer 

 has been in working use for more than 

 a hundred years. 



This instrument, which is merely a 

 watch of especial construction, will only 

 keep a steady rate when it is undis- 

 turbed by irregular shocks or motions. 



No means have yet been found for 

 transporting a chronometer on land 

 without upsetting its regularity, and 

 therefore rendering it useless ; but on 

 board a ship it can be so suspended and 

 stowed as to prevent its being disturbed 

 by any ordinary movements of or in 

 the ship. The accurate time of any 

 place departed from, ascertained by as- 

 tronomical observations, can therefore 

 be carried about on board ship for con- 

 siderable periods, and by comparison 

 with the local time, also determined by 

 sextant observations of the heavenly 

 bodies, at any required spot on the coast, 

 the difference of longitude is at once 

 obtained with very small limits of error 

 when a number of chronometers are 

 employed. These two simple yet mar- 

 velous instruments — the sextant and 



