92 EXPLORATION GEOPHYSICS 



tical intensity component Z. The former is effective on a compass or a 

 horizontal magnetometer, and the latter on a vertical magnetometer or dip 

 needle. 



The horizontal intensity can be projected on the astronomic north 

 direction, which amount is called X, and on the astronomic east direction, 

 which amount is called Y. The angle between the magnetic meridian and 

 the astronomic north is the declination D. The angle between the horizontal 

 intensity vector (or a horizontal plane) and the total intensity vector is the 

 dip (or inclination), symbol I. 



The Magnetic Elements. — The quantities D, H and I are called the 

 magnetic elements. The following formulae relating them to each other 

 need no demonstration : 



X = HcosD; Y = HsmD; Z = H tan I = T sin I 



H = VX^+Y^ = T cos I; T = VX^ + Y^ + Z"" = VTPTZ^ ,,,o^ 



H z ^ y 



T = -;tan/=^g.= . ; tan D = -^ 



cos / H V A ^ + Y^ X 



The U. S. Coast and Geodetic Survey measures the magnetic elements at 

 various stations throughout the United States. Three stations for Colorado 

 are given below. 



COLORADO 



U. S. Government Magnetic Stations* 



Pueblo: Lat. 38°14' Lon. 104°38' 

 Survey 1899 as of l-l-'25 

 Dec. 13°24' E.I. 67°04' 

 H = 0.2247 gauss. 



Fountain: Lat. 38°4r Lon. 104°42' 

 Dec. 14°26'E.I.67°ir 

 H = 0.2222 gauss. 



Canon City : Lat. 38°26' Lon. 105° 14' 

 Dec. 14°2r E.I. 66°30' 

 H = 0.2277 gauss. 



The horizontal intensity is recorded in gauss, 1 gauss being the strength 

 of a magnetic field which acts on a unit magnetic pole with a force of 1 

 dyne. The unit more usual in the field surveys of geophysical prospecting 

 is the gamma (-y) or 1/100,000 of a gauss (10~^ gauss). 



* From Government Magnetic Declination Tables and Isogonic Charts, C. & G. 

 Survey 105, and 36th Ann. Serial Publication of U. S. Coast and Geodetic Survey, 

 Results of Magnetic Survey in 1915-1926. 



