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SECTION I. 



-:o: 



TABLES OF DETERMINED POSITIONS, THE AUTHORITIES, 

 THE VARIATION OF THE COMPASS, ETC. 



The first portion of this volume consists of a selection of the principal 

 geographical points established in the surveying operations, and appended 

 to them are some notes, which will sufficiently explain their nature. In 

 former editions we were led to discuss more fully the merits of the various 

 authorities and the discrepancies between them, which were often con- 

 siderable in amount. Owing chiefly to the extension of the telegraphic 

 system, these differences have been so overcome, and such minute exact- 

 ness attained, that whatever notes are still retained on this topic, must be 

 taken rather as subjects of curiosity than of practical utility. The names 

 of the original observers are also retained in many places in the Tables, 

 more as a record of their services, than as authorities for the positions now 

 given, as may be gathered from the above remarks. 



It is manifestly impossible that the seaman in the ordinary pursuit of 

 his calling can hope to improve what has cost so much labour, with such 

 refined appliances. The Secondary Meridians, and many other points in 

 the Geographic Tables which follow, may be taken as points of departure 

 by which he may correct his reckoning and rate his chronometer ; and the 

 explanatory notes appended will serve to give him confidence, and afford 

 information upon this important section of hydrography. 



Where telegraphic communication renders it possible, the true longitude 

 of a station may be very accurately ascertained by comparing by signal 

 two chronometers, whose errors are known, one at each end of the line. 

 Differences of longitude may thus be measured, step by step, from the 

 prime meridian. Since 1873, the United States Hydrographic Office has 

 determined by this method the positions of the chief points of the New 

 World, including the West Indies, and some of the results are given in the 

 following Tables. 



Secondary Meridians are printed in larger characters, thus: Pobtsmouth, 

 Bbussels, Pabis. 



