5L4 
MR. A. W. RUCKER AND DR. T. E. THORPE ON A MAGNETIC 
The terrestrial curves were calculated from the formulfe 
c = 50°-066 + [77°(D85 - H) - 0°-42 sin (1000^ H - fGo)}, 
I = - {0 14097 + (X - 5-) + 0. 
H being considered as a mere numeral. 
From these, if H is given, c is known, and then the latitude in which the corre¬ 
sponding line of ec|ual Horizontal Force cuts any given meridian of longitude is easily 
found from the second formula. 
In like manner, if I and X are given, H can be found by successive approximation, 
the convergence of the values being in general very rapid. 
Table VI. contains the values of the Horizontal Forces given by these formulc© at the 
points'where whole degrees of latitude and longitude intersect. 
From this table we deduce the calculated values of H at the central stations, 
which are in sufficiently good accord with observation. 
District. 
Horizontal force at Central station. 
Difference in terms 
of 0-0001. 
Mean of values observed 
in district. 
Calculated. 
I. 
1-6011 
1-6020 
— 9 
II. 
1-6519 
1-6519 
0 
in. 
1-7313 
1-7313 
0 
IV. 
1-8070 
1-8068 
“I" 2 
V 
T676] 
1-6770 
- 9 
VI. 
T7083 
1-7082 
+ I 
VII. 
1-8206 
1-8211 
— 5 
VII]. 
1-7197 
1-7184 
+13 
1 IX. 
1-7863 
1-7854 
+ 9 
The Table on p. 515 gives a number of points on the district and terrestrial curves 
respectively. The upper number in each square corresponds to the terrestrial curve 
calculated from Table VI. The curves are exhibited on Map 2. The district curves 
are dotted. The terrestrial curves are drawn in continuous red lines. 
