Or thus: 



computing Magnetic Declination. 513 



sine 20° 0' . . . . 9534-05 * 



sine 1°0' . . . . 824186 



sine 20' ... . 777591 

 Throughout the computation we shall consider it useless to 

 subtract the radius, which only leaves the result without the 

 Jirstjigure. The middle sine 1° is obtained thus : 



91° (as above) -90°= 1°. 



(4.) Lat. and long, of S, magnetic pole. 



S. lat. 75^ 5': comp. 1 i° 55'. 

 E. long. 155°. 



(5.) Obtain the value of yyh in the same way as in item 

 third. 



155° -90° = 65°. 

 sine 14.° 55' ... . 941063 

 sine 65° 0' ... . 995728 



sine 13°29' . . . . 936791 



(6.) Obtain the value of zzc thus : add, or subtract, as the 

 case may be, the value o^ zza^ found in item third, to the com- 

 plement of latitude or rtc. 



38° 3l' + 20' = 38° 51'. 



(7.) Allow 20" to every degree 'without the orbit of the 

 magnetic poles, for the variation of intensity, and 1' to every 

 degree within the orbit. 



38° 5 1'x 20" =12' 57" 

 20° 0' X 1' =20' 0" 



32' 57" 



(8.) Ol)tain the distance zzyy thus : 



zza + ah + hyy ; 

 or 



1 80° + 20' + 1 3° 29' = 1 93° 49'. 



(9.) Correct for the earth's rotundity in latitude, thus : 



ah^ : zzyy'^ : : 32' 57" : 38' 12". 



These minutes are for every ten degrees of the distance azc; 

 hence 



38° 51' X 38° 12'-?-10 = 2° 28'. 



This number is used for the correction of latitude, thus: 



20° + 2°28' = 22°28'. 



Phil. Mag. S. 3. No. 239. Suppl. Vol. 35. 2 L 



