90 



is then described. For the isoclinal lines, four groups were formed : 

 the first consisting of forty stations in Singapore, Borneo and 

 Java; the second of thirty stations in Sumatra; the third of thirty 

 stations at sea ; and the fourth of thirty of the principal stations in 

 the Archipelago. 



The first gave for the latitude of its central station 6° 17' south ; 

 longitude 108° 55' east; and 9T0V'5 south dip. These thirty sta- 

 tions form so many equations of condition, and were combined by 

 the method of least squares, as practised by Colonel Sabine in his 

 Magnetic Survey of the British Isles: from the final equations it was 

 found that the dip increased 1''940 for each geographical mile per- 

 pendicular to the isoclinal lines, and that the direction of the latter 

 is from north 86° 06' east to south 86° 06' west. 



In the Sumatran group the latitude of the central station was 0° 08' 

 south; longitude 100° 31' east; dip 16° 36'-6 south ; the rate of in- 

 crease of dip being 2'°021 for each geographical mile perpendicular 

 to the isoclinal line, the direction of which is from north 83° 28' west 

 to south 83° 28' east. 



In the third, or sea group, the latitude of the central station is 

 2° 38'; longitude 110° 05' east; dip 9° ] l'-7 south; the resulting 

 equations give the isoclinal line running from north 87° 26' west to 

 south 87° 26' east; and the dip in the line at right angles to it in- 

 creases at the rate of l''993 for each geographical mile. 



The last or general group of thirty of the principal stations 

 in the Magnetic Survey give the resulting equations at a mean 

 latitude of 0° 09° north, mean longitude 101° 44' east, and mean 

 dip 14° 40'-4 south ; the isoclinal line running from north 86° 39' 

 west to south 86° 39' cast ; and the dip in the line at right angles to it 

 increasing at the rate of 1''953 for each geographical mile. 



Nearly the same method has been pursued in laying down the 

 lines of equal horizontal force. In one group this line forms an 

 angle of north 67° 48' west to south 67° 48' east with the meri- 

 dian, and the line perpendicular to it proceeds at the rate of 

 '0008249 for each geographical mile, the central station being in 

 latitude 0° 38'-5 north, and longitude 102° 26' east. In another 

 group at the central station 3° 40' south, and 106° 33' east, the 

 line of equal horizontal force forms an angle of 60° 57' with the 

 meridian, and increases at the rate of '0005855 for each geographi- 

 cal mile on the perpendicular line. 



The lines of declination have been laid down with reference to 

 the declination at each station individually, the particular form of 

 the lines rendering the method of grouping inapplicable. 



Toted force. — The total force was not determined directly, but by 

 the formula f=h . sec ^, where h is the horizontal intensity, S the 

 dip, and / the total force; and as the dip changes rapidly, but 

 regularly, whilst the horizontal intensity changes slowly, but irregu- 

 larly, it is evident the lines of total force will follow in some de- 

 gree the regularity of the intervals between the lines of dip. 



The total force for each station was determined by the above 

 formula, and equations of condition, combined by the method of 



