328 CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 
2nd Group. — Sumatra contains thirty stations ; the resulting equations give — 
o / 
At a mean latitude . = 0 08 South. 
At a mean longitude. =100 31 East. 
And mean dip . . = 16 36' 6 South. 
^= + 0-230; z/=+2-008 
r= + 2''02I and m=83° 28'. 
We have therefore, the isoclinal line in Sumatra forming an angle of N. 83° 28' W. 
to S. 83° 28' East, and the line at right angles to it increasing at the rate of 2''021 ; 
and as the central station, through which this line passes, is only 8 miles to the south 
of the equator, we may assume this to be the rate at which the dip increases at nearly 
right angles to the terrestrial equator in longitude 100° 31' East. 
3rd Group. — From the third group we have thirty observations taken at sea. 
The resulting equations give — 
o I 
At a mean latitude . = 2 38 North. 
At a mean longitude =110 05 East. 
And mean dip . . . = 9 11*7 South. 
+0-089; ?/= + 1-991 
m= 87°26'; r= + l'-993. 
We have, therefore, the isoclinal line running N. 87° 26' W. to S. 87° 26' East, and 
the line at right angles increasing at the rate of +l'-993 for each geographical mile. 
4th Group. — In the fourth group are contained the principal stations in the Mag- 
netic Survey, extending from the 80th to the 125th degree of east longitude, and 
from 16^° North to 12° South. 
From the resulting equations we obtain — 
o I 
At a mean latitude . = 0 09 North. 
At a mean longitude =104 44 East. 
And mean dip . . = 14 40*4 South. 
^=+0-114 3/= + 1'-947 
m= 86°39'; r= + l'-953. 
The isoclinal line runs therefore N. 86° 39' W. to S. 86° 39' East, and the line at 
right angles to it increases at the rate of l'-953 for each geographical mile. 
On the Lines of Equal Horizontal Intensity. 
For the lines of equal horizontal intensity the equations of condition have been 
combined in the same manner by the method of least squares. 
From one group of forty stations 
o / 
At a mean latitude of ... . 0 38-5 North. 
At a mean longitude of . . . . 102 26 East. 
j;=+ -0003 12076; ?/= +-000765277 
r= + -0008249; m=67° 48'. 
