MAGNETIC OBSERVATIONS. 



41 



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



<p = 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. 

 <5 = sun's declination; positive when north. 

 Then we have 



a + a' 



tan M= 

 tan A = 



+ dt + x 

 tan 8 



cos t 



tan t cos M 

 sm (<£ — M) 



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

 than 180°. 



Magnetic declination = R' -\- = (p — p') -f A — 180° — - ~t*L 



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 



i (p + p') " 



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° — E "+ E '" 



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. 



V 



Vernier . 



Vernier . 



Vernier . 



s Mean . 



12° 23' 30" 



75° 2 5' 3°" 

 74 55 3° 



(1) Scale erect 



(2) Scale inverted 



(1) -(2) = a 



78 d .o 

 80.3 



— 2 -3 



■5 



<L> 



ft " 

 O 



Transit of sun's 



CD 



i st limb 

 2d limb 



Mean 



8 h i4 m 28" 

 15 28 





75 IO 30 



8 14 58.0 



March, 1872. 



