

138 ASTRONOMICAL AND GEODETIC WORK 



PREFACE. 



I. Classes of OBSEE.vATI0^^g. 



The observations for tlie determination of the latitude and longitude were of two classes — the 

 first being those which were made at primary stations with the largest instruments that could 

 be conveniently transported by land ; the second, those made at places of less importance, deter- 

 mined by reflecting instruments, and by the transmission of chronometers from the nearest 

 primary stations, or by flashes of gunpowder^ observed simultaneously. Eighteen stations of 

 the first class were established across the continent at the following places, named in their order, 

 from west to east: 



1. Camp Riley, near initial j)oint, on the Pacific coast ; 2. Junction of the Gila and Colo- 

 rado ; 3, On the Colorado, where the line leaves that river ; 4. Quitobaquita ; 5. Los Nogales, 

 near 111th meridian of longitude ; 6. San Bernardino, near parallel 31^ 20'; 7. San Luis 

 springs, near parallel Sl*^ 20' ; 8. Agua del Perro ; 9, Espia ; 10. Carrizalillo, near parallel 

 31^ 4V ; 11- Erontera; 12. San Elceario ; 13. Initial point of boundary on parallel 31^47'; 

 14. Caiion where the San Antonio road leaves the Rio Bravo ; 15. Presidio del Norte ; 16. Eagle 

 Pass, (Fort Duncan ;) 17- Ringgold Barracks ; 18. Mouth of Rio Bravo. 



I do not enumerate the very many points of lesser importance determined by the sextant and 

 chronometer^ as no special notice is required of the means used in their determination. It may 

 be as well to state, however, that in all cases w^here observations were made for latitude with 

 reflecting instruments, stars were taken, both north and south, and at altitudes as nearly equal 

 as could be obtained ; and when local time was obtained by these instruments, stars were taken 



r 



botli east and westj of nearlj equal altitudes, and as near tlie prime vertical as they could be 

 found. In this way we attempted to avoid the errors arising from refraction and from the 

 eccentricity of the instrument. 



In some cases, where I had direct comparisons with results obtained by the large instrument, 

 I ascertained that the latitude of a place determined by the Gambey sextant in the proper hands, 

 in a single night, might be relied upon to within 3" or 5"; and if the observation was repeated 

 for two nights, the result might be relied ux^on to be within 2" of the true position. 



II. LOXGITUDE. 



The longitudes of 1, 2, 4, 5, 10, 11, 12, 15, 16, 17, 18, were obtained by observations on 



the transit of the moon and moon culminating stars, with a telescope of forty-six inches' focal 



length, by Troughton & Simras, of London, and with a smaller one of thirty-six inches' focal 



length. Occultations were observed, whenever practicable; but, owing to the impossibility 



of obtaining corresponding observations, they proved of little value; indeed, but few were 



observable. Occultations are no doubt of the greatest value where corresponding observations 



at some established point are observed ; but for the general purposes of a survey, conducted in 



distant regions, they are not a sure reliance. They occur too seldom ; and to see the instant of 



immersion and emersion of a star requires great steadiness in the telescope, which in the field 



can be rarely obtained, owing to the difficulty of protecting the telescope from the wind, and of 

 getting a firm support. 



