IV 
must be the Total Error of the compass in the ship, or, in other words, the 
amount combined of the variation of the compass at the ship’s place, together 
with the deviation for that point of the compass to which the ship’s head may 
then be directed. To ascertain the deviation we require, therefore, to know 
the variation at the geographical position of the ship—this can be conve 
niently obtained from the charts published by the Admiralty* ; to the Total 
Error obtained apply the variation with its opposite name—if of unlike names 
take the difference, putting to the remainder the name of the greater—but if of 
like names take the sum, and put the common letter to the result. This result 
will be the deviation required. 
Example— On October 11, 1894, a vessel in lat. 49° 0' N., long. 10° 0' W., 
steering East, at 8h. 36m. a.m. apparent time observed the bearing of the Sun 
by the azimuth or standard compass to be S. 39° 30' E., or reckoning from the 
north, N. 140° 30' E. Required the deviation, the variation as shown on the 
Chart of Magnetic Curves being 21° W. 
Sun’s true bearing by tables 
Sun’s bearing by compass 
Total Error determined in ship 
Variation from the chart with opposite name 
Deviation, ship’s head East 
Construction of the Tables. 
The tables are computed from the following 
formula :— 
Given P Z = Co-latitude. 
PA = Polar distance. 
P = Hour angle (for the Sun, the apparent 
time from noon) 
to find Z = Azimuth. 
(1) Let a = 
PZ + ZA 
(2) then tan = cot ~ sec a cos h, when a is less than 90° 
Z i 
or tan (180 — = cot ~ sec (180 — a) cos b, when a is greater than 90° 
z z 
7 ^ A P 
(3) tan = cot —cosec a sin b 
Z i 
b = 
PZ ~ ZA 
2 
N. 125 34 E. 
N. 140 30 E. 
14 56 V/. 
21 0E. 
6 4E. 
*Cliart of the Curves of Equal Magnetic Variation, 1895, No. 2598. See also Admiralty 
Manual for ascertaining and applying the Deviations of the Compass, sixth edition, 1893, page 5, 
art. 16, and page 108, art. 11 ; sold by J. D. Potter (Agent for the Admiralty Charts). 
31, Poultry, and 11, King Street, Tower Hill, London, and all other agents for the sale of 
Admiralty Charts. Price 3 s.