THE COMPASS-NEEDLE ON THE DEVIATIONS OF THE COMPASS. 
1&6 
Compass cards used. 
Deviation 
caused by 
A 
B 
c 
D 
E 
P 
G 
H 
K 
inches. 
0 1 
-0 9 
-30 28 
O i 
+0 1 
O 1 
-0 1 
+6 20 
O ! 
0 1 
0 1 
a i 
-0 2 
- 3 ... 
' 
Bar 
^ magnet , 
placed 
endways. 
( 
-0 8 
-0 3 
-0 6 
Single needle 
6 ... 
-0 19 
-30 47 
+0 8 
+0 9 
+0 19 
-1 25 
-0 2 
+0 1 
-0 1 
.12 ... 
-0 9 
-31 45 
+0 10 
+ 1 4 
+0 11 
-5 38 
-0 2 
+0 16 
+0 5 
Compound needles • 
, 
-0 5 
-0 12 
-35 17 
-36 26 
+0 1 
+0 4 
-0 38 
0 0 
-0 9 
+0 8 
+0 55 
+0 39 
+0 11 
+0 1 
-0 8 
-0 2 
-0 6 
+0 6 
3 ... 
• 
Bar 
magnet 
placed 
sideways. 
-0 4 
+24 11 
-1 12 
-0 36 
+0 15 
-0' 59 
+0 20 
+0 1 
+0 12 
Single needle 
6 ... 
+0 2 
+24 34 
-1 0 
-0 33 
+0 25 
-4 30 
-0 22 
+0 2 
+0 10 
,12 ... 
-0 16 
+25 47 
-1 12 
-0 45 
+ 1 44 
-9 56 
+0 20 
+0 40 
-0 32 
Compound needles ■ 
r 3| ... 
... 
+0 56 
+0 15 
+35 44 
+36 16 
-0 22 
+0 1 
-0 25 
-1 46 
-0 21 
+0 53 
+0 19 
+ 1 16 
-0 4 
-0 1 
+0 12 
+ 0 1 
+0 18 
+0 5 
Compound needles 
7 \ ... 
1 
Soft iron 
-0 13 
+ 05 
-0 5 
-7 48 
-0 13 
-0 1 
-0 1 
-0 37 
+0 3 
Single needle 
7 ^ ... 
cylinders. 
1 
-0 20 
- 0 1 
-0 18 
-8 3 
+0 7 
+0 5 
+0 1 
+2 47 
-0 17 
The following are briefly the results of the experiments : — 
1. Deviations produced by magnets in the same horizontal plane with the compass, 
and at distances of 18^ and 19f inches from the centre of the compass: — 
With the 3-inch single needle these deviations are nearly semicircular. 
With the 6-inch single needle, and still more strikingly with the 12-inch single 
needle, a large sextantal deviation is introduced, and the semicircular deviation is 
increased, the sextantal deviation and the increase of the semicircular deviation being 
proportional approximately to the square of the length of the needle. 
A striking diflerence will be observed in the appearance of the curves of deviations 
caused by a magnet placed endways and the curves of deviations by a magnet placed 
sideways. In the first case, the semicircular curve is broadened and flattened by the 
introduction of the sextantal deviation. This it will be seen arises from the coefficients 
F and B having the same signs. In the second case the semicircular curve is narrowed 
and peaked. This arises from the coefficients F and B having different signs. 
These peculiarities, as will be seen in the sequel, agree Avith the mathematical 
deductions. 
In the deviations of the Admiralty Standard comjyass cards, whether the magnets were 
placed endways or sideways, the sextantal deviation almost entirely disappeared. 
2. DeAiations produced by magnets above or below the level of the compass: — 
The mathematical investigation shows that when the difference of level is less than 
half the horizontal distance, the semicircular deviation is increased by increasing the 
length of the needle, when greater, diminished ; and with this the observations agree, 
the following being the results. 
With the 3-inch single needle the deviation was in each case nearly semicircular. 
With the 12-inch single needle raised 20 inches above the magnet, when the hori- 
zontal distance was 6-^ inches, the deviation was nearly semicircular, showing only a 
very slight tendency to the introduction of a sextantal part. As the horizontal distance 
increased, the proportion of the sextantal to the semicircular deviation increased. 
With an Admiralty Standard compass there was no sextantal deviation. 
3. DeA'iations produced by soft iron at the level of the compass : — 
