104 



MAGNETIC OBSERVATIONS 



TABLE III. -TABULATION OF RESULTS FOR DETERMINATION OF SECULAR 



* Mean of three stations, giving Hog Island one-half weight. 



From the above results of magnetic declination, it is found that the 

 average secular change in the Bahama Islands, during the past six decades, 

 has been 3.5' per annum, west declination having been increased annually by 

 this amount and east declination decreased by same amount. 



The results of dip and intensity are too few as yet to deduce safely the 

 secular change in these elements. 



DIRECTIONS FOR THE MAGNETIC WORK OF THE BAHAMA EXPEDITION.- 



A. Astronomical Observations. 



These will consist of two sets of sun observations made preferably both 

 in the morning and afternoon, not necessarily on the same day, however, for 

 obtaining the true azimuth of uuirk or marks used in the magnetic observations 

 and the correction on local mean time of the watch used. By securing sun ob- 

 servations both morning and afternoon (two sets each time), at the same sta- 

 tion, any error that may be due to latitude used in the computation is eliminated 

 in the mean of the a. m. and p. m. results. If this cannot be done and it is 

 not possible to obtain the latitude from good maps, it is necessary to obtain 

 the latitude by circum-meridian altitudes of the sun at noon. In these ob- 

 servations the tripod should be mounted on the tripod pegs, packed in the 

 tent. 



The azimuth observations should lie made preferably between 2 and 4 

 hours from local noon, i. e., 8-10 a. m. and 2-4 p. m. local time, and any one 

 set should not cover a time interval of more than 8-10 minutes. (See App. A, 

 Mag'n Decl'n Tables 1902, and App. 8, Eep. for 1881.) 



It is supposed that the observer will use his own watch for these obser- 



- Furnished by the Superintendent of the U. S. Coast and Geodetic Survey. 



