286 
COMMANDER JOHNSON’S MAGNETIC EXPERIMENTS 
4th. That it never should be placed on a level with either the ends of horizontal of 
perpendicular bars of iron. 
5 th. That the extreme ends * of an iron vessel are unfavourable positions, owing to 
magnetic influences, and that the connecting-rods, shafts, &c. of the machinery, being- 
moveable quantities, renders the centre objectionable, independently of the position 
of the great iron funnel. 
6th. That no favourable results were obtained by placing the compass below the 
deck, nor on a stage over the stern. 
7th. That at the positions G, 20^ feet above the quarter-deck, and I, 13^ feet 
above the same level, and about one seventh the length of the vessel from the stern, 
the deflections of the horizontal needle were less than those which have been ob- 
served in some of His Majesty’s ships ; but in order to prove whether this or a nearer 
approach to the deck may be fixed upon as a proper place for a steering-compass, the 
following experiments should be tried. 
The horizontal deflections at the different points as before, both with the fires 
lighted and extinguished'!''. The dip and magnetic intensity to be ascertained both 
when the vessel’s head is at the true magnetic north and south, and likewise when it 
is at the north and south indicated by the needle at the position of observation on 
board ; and further, that the vertical and horizontal vibrations should be determined 
when the vessel’s head is towards the east and west ; and it would tend to accuracy 
if these were obtained when the vessel was aground. 
It would be desirable to repeat some of the experiments after the keel of the vessel 
has remained some time in the line of the magnetic meridian, and afterwards to com- 
pare them with those which might be obtained after her head had been kept to the 
east and west for the same length of time. 
The vibrations of the needle, and a few bearings that could be quickly and accu- 
rately obtained after the vessel has been in motion through the water, would be de- 
sirable. The observations which were commenced on a needle placed in the centre 
of an iron sphere (3 feet in diameter) should be completed. 
As it is known that iron when heated to a red colour attains an extraordinary mag- 
netic power, it would be advisable to observe whether a very delicately constructed 
needle suspended in the position where the compass is to be placed, be affected at 
such a distance from the furnace on board, and this should be accomplished when 
the vessel is aground. 
Similar experiments to those tried on the quay in Tarbert Bay might be repeated 
with advantage after the vessel has been in motion through the water, and needles 
ascertained to be of different intensities might be used. 
Besides ascertaining the horizontal deflections of the needle produced by the local 
* This remark does not apply to the bowsprit, as the curious results obtained at C, which have been already 
noticed, show that further observations are requisite. 
t Especial notice of the compass on the bowsprit should be taken during the several revolutions of the vessel. 
