CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 329 
The line of equal horizontal intensity forms an angle of N. 67° 48' W. to S. 67° 48' E. 
with the meridian, and on the line perpendicular to this the horizontal intensity 
increases at the rate of ’0008249 the geographical mile. 
From a second group of seventy-seven stations, comprising all those at some 
distance to the south of the line of maximum horizontal intensity, we obtain 
o / 
At a mean latitude . . . . = 3 40 South. 
At a mean longitude . . . . =106 33*0 East. 
x=-f -00028453; ^= -1-’00051 17 
m= 60°57'; r= + -000585439. 
The line of equal horizontal intensity forms an angle of N. 60° 57' W. to S. 60° 57' E. 
with the meridian, and the horizontal intensity on a line perpendicular to this 
increases at the rate of ’0005855 the geographical mile. 
Lines of Total Intensity. 
The total intensity has been determined by the formula A=/'cos 
„ A , . 
or J—~ — ?=Asec6, 
'' cos 0 ’ 
where h is the horizontal intensity, 
^ the dip, 
f the total intensity ; 
eighty equations of condition of the following form, 
ax-\-hy=f—f, 
have been combined by the method of least squares, so as to assume the form of the 
two following equations : — 
a^x-\-abyr=a{f—f) 
ahx-\-Wy—h{f-f), 
a and b being the coordinates of distance in longitude and latitude from the mean 
station for any station, and f its intensity; /*the mean total intensity =8’745. 
Then by adding together the equations of condition for each station, we obtain 
the following : — 
r= — ’001073, 
u= 85° 36', 
at a mean latitude of 3° 05' S. and a mean longitude of 106° 47' E. 
The angle which the line of equal total intensity forms with the meridian is 
N. 85° 36' W. to S. 85° 36' E., and increases southerly at the rate of ’001073 the geogra- 
phical mile. The line of least intensity in that part of the Archipelago over which I 
have carried the Survey, agrees nearly with the line of no dip. On looking over the 
list of stations, we find Sambooanga, Nicobar, and Madras, with the least total inten- 
MDCCCLI. 2 u 
