284 
COMMANDER JOHNSON’S MAGNETIC EXPERIMENTS 
In considering' the contents of these Tables, especially as respects the amount of 
deflection, it is requisite to bear in mind that the needles, being so placed on the quay 
as to avoid the influence of each other, were at different distances from the vessel, 
and not in the same line of bearing ; consequently, when the vessel approached very 
near to the quay with her head or stern, the true magnetic bearing of those parts 
from each instrument was necessarily different : hence the difference of the amount 
of deflection. 
As an iron ball, or disc, when made to approach a magnetic needle from the south- 
east, would produce a different deflection when its centre was above the equator of 
the dipping-needle to that which would occur when it was below, viz. in one case the 
deflection would be to the eastward, and in the other to the westward, due considera- 
tion was given to that circumstance in the examination of the results ; and the fol- 
lowing Table, showing the elevation of the vessel’s stem and stern, as related to the 
level of the needles, and likewise the depression of the keel below their level, may be 
useful in reviewing this subject. 
The observations having been made from about an hour and a half before to an 
hour after high water, a mean level has been taken in the construction of this Table ; 
and it is to be understood that the tide neither rose three feet above nor fell three 
feet below this mean level during the experiments in question. 
Part of vessel 
nearest to the 
instruments. 
Dipping-needle. 
Compass O. 
D. 
E. 
V. 
Upper part 
of stern. 
Keel below 
axis. 
Upper part 
of stern. 
Keel below 
pivot. 
Upper part 
of stern. 
Keel below 
pivot. 
Upper part 
of stern. 
Keel below 
pivot. 
Upper part 
of stern. 
Keel below 
pivot. 
Head. 
1 foot 
below 
axis. 
12| feet. 
0| feet 
below 
pivot. 
14J feet. 
4tj feet 
above 
pivot. 
9 feet. 
4J feet 
above 
pivot. 
9 feet. 
4§ feet 
above 
pivot. 
9 feet. 
Stern. 
Iron rails 
over stern 
feet 
above axis. 
12| feet. 
Iron rails 
over stern 
2 feet 
above 
pivot. 
14^ feet. 
Iron rails 
over stern 
7 feet 
above 
pivot. 
9 feet. 
Iron rails 
over stern 
7 feet 
above 
pivot. 
9 feet. 
Iron rails 
over stern 
7 feet 
above 
pivot. 
9 feet. 
It will be noticed, that as the stern approached the instruments the iron rails over 
the tafrail of the vessel were above the level of the compass O ; and when the head 
approached, the upper part of the stein was rather below that level ; but with respect 
to the other needles D, E, V, both the upper parts of the stem and stern were above 
their level. 
The above remarks, however, only relate to those portions of the vessel that ap- 
proached nearest to the instruments ; but in looking to the probable centre of the 
entire mass of iron in the Garryowen, and the different heights of the compasses on 
the quay, it is impossible to attribute the opposite deflections which occurred to the 
difference of elevation of the centre of the mass that could take place by the rise and 
fall of the tide during the observations. 
The conclusion, therefore, which I have come to is, that the deflections alluded to 
were caused by the magnetic influence of the iron in the vessel ; the combined effect 
