MAGNETIC OBSERVATIONS. 



41 



t = the sun's hour angle at the pole. 



< = latitude of the place of observation; positive when north of the equator. 



A = azimuth of sun's centre at the time of its transit over the vertical wire : the 



azimuth being counted from the south around by the west. 

 S = sun's declination ; positive when north. 

 Then we have 





tan M = 



tan 



tan A = 



cos t 



tan t cos M 

 sin (<p M ) 



where A is to be taken greater or less than 180 according as t is greater or less 

 than 180. 



Magnetic declination = R' -\- | (p p') -f A 180 ^fc^ 



in which the declination is east if its sign is positive ; west if its sign is negative. 

 The reading of the magnetic axis of the magnet is 



which we will designate by c. It should be constant. Then, if at any station the 

 magnet has only been observed with its scale erect, if c is known the observation 

 may be reduced by the formula 



Magnetic declination = R' + d(p c) -f- A 180 



to 



The following example shows fully the form employed in recording and reducing 

 the observations. 



Magnetic Declination. 



Station, Acapulco, Mexico. Date, May 30, 1866. Portable Declinometer, D. 22. Magnet C. 32. 



Observer, WM. HARKNESS. 



Circle readings. 



Reading of magnet. 



6 March, 1872. 



