276 
COMMANDER JOHNSON’S MAGNETIC EXPERIMENTS 
obtained with sufficient exactness, from the cause I have already mentioned, viz. the 
too near proximity of certain masses of iron. 
In most cases “ the plane of no deviation” has been found in the direction of the 
keel ; there are, however, exceptions to this ; and the little approximation to such a 
line that may be deduced from the observations at A, shows that in the Garryowen 
at that position the line of no deviation would be oblique to the keel. 
Comparing- the results obtained at positions A and B, and referring to the plan of 
the vessel, which shows the variety of positions in which malleable iron is placed, — 
such as in the beams, ribs, sheathing, rails, stanchions, &c., in all of which magnetic 
influences might exist, I determined to remove the compasses further from the deck, 
and consequently further from the particular portions of the iron work which ap- 
peared to me to affect them. 
These observations, while they proved that the distance of 5 or 6 feet from any iron 
work was not sufficient for placing a compass at A and B on board the Garryowen, 
they at the same time clearly demonstrated the necessity of observing the compara- 
tive effect produced on the dip and magnetic intensity by the vessel ; for although 
the horizontal deflections of the needle did not appear to be excessive at position A, 
yet it was evident that its directive power was affected. 
The very unfavourable weather that prevailed prevented me from making all the 
observations on the dip and intensity that I desired, or those suggested, as both the 
vertical and horizontal vibrations, as well as the dip, should be determined with the 
vessel’s head at the true magnetic north and south, and likewise at the north and 
south points indicated by the needle at the position on board ; besides which similar 
observations should be made with the vessel’s head at east and west, the object being 
to ascertain how far the directive power of the needle may be affected at certain parts 
of the vessel. The observations on the dip and intensity which I was enabled to 
make are registered in the following Tables VI. and VII. 
