344 
IVIE. EVAXS ON THE DEVIATION OE THE CO^^IPASS 
O i 
Assistance . 
. 1820 tons, iron beams . 
+ 3 
36 
Trident . 
. 850 tons, wood beams . 
+ 3 26 
Industry 
638 tons, h'on beams . . 
+2 
40 
Oberon . 
649 tons, wood beams . 
+ 3 
27 
Bloodhound 
378 tons, iron beams . 
+ 3 
40 
Sharpshooter . 
. 503 tons, wood beams . . 
+ 4 32 
To establish the effect of iron beams more fully, I carried out a series of experiments 
on board H.M.S. Supply ; and from the results it is to be inferred that their magnetic 
character is identical with that of the hull of the ship ; being dependent, as regar-ds 
polarity and permanency of magnetism, on the dhection of the ship when building, and 
the nature of the iron. 
The uniformity of the quadrantal deviation of the compass, when placed in various 
positions and at different heights fi:om the deck, is thus shoum : — 
ft. in. 
In middle line of the ‘ Supply,’ 28 feet from stern, 5 0 high from deck . 
,, 19 ,, 5 2,,.. 
12 „ 3 10 „ 
ft. in. 
Placed 22 feet from stern, 3 feet from middle line of deck, and 3 9 high 
+2 38 
r+2 15 
1+2 37 
+2 55 
+2 56 
Placed 13 feet from stern, 8 feet from middle line of deck, 2 feet fifom the 
iron top side, and 1 foot 8 inches high 
+2 13 
The polar-magnet or semicircular deviation varied under these conditions from 15° 
to 45°. 
Elevation of the compass above an ordinary height fr’om the deck necessarily dimi- 
nishes the amount of quadrantal deviation, as in mast -compasses * ; brrt uithirr the 
ordinary limits, or from 2 to 6 feet from the deck, the foregoirrg examples show that the 
value is not affected. Another example was also afforded hr H.M.S. Assistance, where 
the raising the standard compass from 4 feet 5 inches to 6 feet 4 mches above the iron 
beams, which .dimirrished the “semicircular” deriation from 17° 40' to 15°, had rro per- 
ceptible effect on the “quadrantal,” the values of D bemg respectively +3° 35' and 
+ 3° 36'. 
Mast-compass, Standard compass, 
45 feet horn deck. 5 feet from deck. 
r Eiver Thames -fl 32 -f 4 21 1 
* Great Eastern J Portland d-l 30 -t-4 44 t Coefficient D. 
Holyhead -f-l 0 +4 sj 
H.M.S. Trident, mast-compass elevated 48 feet from the deck, D — 1° S': this is the only example that 
has come under my investigation of a minus sign. It is the effect -which vrould be produced if the soft iron 
can be considered as collected in two masses, — one near the bow, the other ne;u’ the stern. 
