CAPTAIN ELLIOT’S MAGNETIC SURVEY OF THE INDIAN ARCHIPELAGO. 331 
The line of equal inclination forms an angle of 86° 40' with the meridian, and 
changes in a direction perpendicular to it at the rate of l'‘970 the geographical mile. 
The line of equal horizontal intensity forms an angle of nearly 65° with the meri- 
dian. For the position of the central line of maximum horizontal intensity, I have 
taken stations north and south of this line having the same horizontal intensity, and 
have divided the distance between them ; and in laying down the rate of decrease of 
the horizontal intensity, I have fixed upon the Java group, which has the greatest 
number of stations, as decreasing at the rate given me by the equations of condition ; 
but since the horizontal intensity at the Cocos shows a much more rapid decrease 
when at some distance from the line of maximum horizontal intensity, I have gra- 
dually increased the rate of progression to the Cocos, and decreased it in the same 
proportion northwards from Java to the line of maximum horizontal intensity. 
The lines of total intensity are laid down in strict conformity with the result- 
ing equations of condition. The direction of the lines is between 85° and 86°, and 
proceeds at the rate of ‘001073 the geographical mile. There is much less difficulty 
in laying down the total intensity lines than the lines of horizontal intensity ; for the 
former, and the rate of progression in a direction normal to them, are functions of the 
dip and horizontal intensity ; and since the dip changes very rapidly, whilst the hori- 
zontal intensity is nearly stationary in the immediate vicinity of the maximum hori- 
zontal intensity, it follows, first, that the line of minimum total intensity will coincide 
nearly with the line of no dip ; and secondly, that its rate of progression, or the value 
of r, will be as uniform and as constant as the lines of dip, for it has been shown that 
over the space of 28° of latitude over which this Survey extends, the dip changes at 
the rate of nearly two miles for every mile of latitude. 
In laying down the lines of variation, I have been guided partly by the equations 
of condition, but to be directed by these alone would lead to very unsatisfactory re- 
sults. I have, in the Java group and in the Eastern group, conformed to the results 
of the equations of condition combined by the method of least squares, whilst in 
the western group I have simply connected together by lines the stations having the 
same variation. 
Explanation of Plates XII. and XIII, 
Plate XII, contains the isoclinal lines or lines of equal magnetic dip and lines of 
equal magnetic declination, as well as the central lines of minimum total intensity 
and of maximum horizontal intensity. 
Plate XIII, contains the isodynamic lines or lines of equal horizontal intensity 
and of equal total intensity ; and in addition, the line of no magnetic declination and 
the line of no dip. 
Pimlico Lodge, JVestminster , 
November 15, 1850. 
