844 Royal Society. 



On arrival at a station, the tent was pitched, and the instruments 

 prepared. These were a six-inch dip circle, a portable decli- 

 nometer, an altitude and azimuth instrument by Robinson, and a 

 chronometer which had been long in use, and was scarcely trust- 

 worthy for a fixed rate. Commencing the following morning, the 

 first instrument set up was the declinometer, and as the suspen- 

 sion-thread was thicl<er than necessary, to obviate the necessity of 

 frequent renewal, the brass weight for removing the torsion was 

 allowed to swing for a couple of hours. During this interval the 

 observations for dip were completed, and by 9 a.m. the collimator 

 magnet was in the Ijox, the altitude and azimuth instrument in rear 

 of it, and in adjustment with it. Sights were then taken with the 

 sextant and artificial horizon for time, and with the altitude and 

 azimuth instrument in connection with the collimator magnet for 

 declination. From 10 a.m. to 11 a.m. observations of deflection at 

 four different distances, and of vibration, were made with the port- 

 able declinometer, and the telescope of the altitude and azimuth instru- 

 ment, for the absolute value of the horizontal force. At noon circuni- 

 nieridional altitudes were observed for latitude, and in the afternoon 

 equal altitudes were taken to confirm the observatioiis of the morning. 

 The instruments were then packed up and sent off to the next sta- 

 tion. This was the system adopted whilst travelling ; but at the fixed 

 stations a great number of additional observations were made of hori- 

 zontal force, dip and declination. 



The method of grouping the results at the different stations for 

 the purpose of drawing the isoclinal, isodynamic and isogonic lines, 

 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 fii-st gave for the latitude of its central station 6° 17' south ; 

 longitude 108" 55' east; and 27° 01'-5 south dip. These thirty sta- 

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

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

 Magnetic Survey of the British Isles: from the final equations itAvas 

 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 



