TERRESTRIAL MAGNETISM. 25 
Note——When the direction of the ship’s head is between N. 45° E. 
and 8. 45° E., or between N. 45° W. and 8. 45° W., formulas (2) and 
(5) should be employed. In the Northern and Southern semicircles, 
between N. 45° E. and N. 45° W., and between S. 45° E. and 8, 45° 
W., formulas (3) and (5). 
The several corrections having been calculated, the observed dip 
(6) requires three corrections— 
1. For Index error (for Fox circle only). 
2. For direction of ship’s head. 
3. For the vertical force of the ship = A. 
3. Total Force. 
Tables of Corrections for deviations of the Total Force, due to 
the direction of the ship’s head, may be computed by the formula— 
R= A'H(scos&+N)cosec@& . . . (7) 
The differences between the several computed values of R’ for eight 
or more equidistant directions of the ship’s head, and the mean of 
the whole = R°, will give the corrections. 
With the values of A taken from the corrections for dip, and the 
value of @ already obtained from the corrected values of 6’, the Total 
Force R can be calculated by the formula— 
R = (R’sin 6°+ AH) cosec@ . . . (8) 
The values of 6° can be taken from those of N = tan 6°. 
When observations of R’ can be observed with the ship’s head on 
or near both East and West by the Fox compass, the mean R’ = R°. 
Then if the value of ~ be observed about the same time from vibra- 
tions on East and West, and also on neighbouring ice, the Vertical 
Force Z can be obtained by the formula— 
R° sin & 
Z= i eh ah ede ae eget ae: 9) 
As the taking the vibrations on ice will occupy a very short time 
compared with the full observation for force, this method is recom- 
mended-when time presses and the ship is in smooth water. 
General Remarks. 
In addition to the Land stations mentioned above, the following 
are selected regions for observations in the ship :— 
(a) Between the Cape of Good Hope and Melbourne, on or near 
the parallel of 40° 8. 
