268 COMMANDER JOHNSON’S MAGNETIC EXPERIMENTS 
bearing of it with the azimuth compass, that was afterwards used at the position A 
on board the Garryowen. 
The magnetic meridian at the station X being determined by this compass, a 
distant object on the land was noted in the line of that meridian, and the theodolite 
in the succeeding observations was placed in the position of the same compass, so 
that in the simultaneous observations between A and X the bearings may be con- 
sidered commensurate with those that would have been obtained by one compass 
only. 
The preliminary observations being complete, I directed the Garryowen to be taut 
moored in the line between the station X and the cone of the mountain, in which 
line of direction was also a remarkable heap of stones on Kilkerran Point, so that 
the vertical wire of the theodolite (placed at station X) bisected these objects, and 
likewise the instrument on the forecastle of the vessel*. These objects on the land 
served also as excellent marks to ascertain if the vessel drifted from her proper 
position, for which purpose thwart marks were likewise fixed upon. 
In order to have the conditions respecting the position of the iron on board always 
the same during the experiments, the Garryowen was moored with extra anchors and 
hawsers, so that her own bower anchors were at the bows, and their chain cables in 
a given place. The tiller, the crane, and boats’ davits (being made of iron) were 
also secured in certain definite positions ; and I am induced to mention these par- 
ticulars, as it will be seen by the experiments how essential their observance is to 
accuracy. 
Anchors having been laid out, and the necessary preparations made for warping the 
vessel’s head to the required points, I thought it desirable previously to fixing the 
compasses in certain positions on board, and swinging the vessel an entire revolution, 
to ascertain the amount of local attraction when the Garryowen’s head was in a 
direction where, in the generality of cases, the deviation had been found to approach 
its maximum, so as in some degree to guide me in the selection of a place for the 
principal observations, or that to which my chief attention was directed, viz. a po- 
sition for placing a steering-compass, and that where the effect of Professor Barlow’s 
correcting-plate might be tried. 
With this view the vessel’s head was warped to the true magnetic west, and the 
deviation ascertained in various parts of her. 
At a position (No. 5) near the fore hatchway, and 5 feet above the deck, 
the deviation of the marked or north end of the needle was . . . 16° E. 
Near the centre of the vessel before the large funnel, and 5 feet above the 
deck, the deviation was 26° 20' E. 
* This instrument was the graduated circle of a surveying-compass, to which I applied cross vanes for the 
purpose of observing the true magnetic direction of the vessel’s head, a plan which I found more convenient 
than using a sextant for that purpose while warping round from point to point. 
