ON AN IRON STEAM-VESSEL. 
279 
It will be sufficient in this place to notice that the dip on Tarbert Island was 
70° 51' with the face of the instrument to the east and west, and that at the position A 
on board the Garryowen, when her head was to the true magnetic north, the dip 
was 78° 25', and when her head was to the south the dip was 73° 54'. The dip 
observed at several positions is inserted in Table VI., and the vertical vibrations for 
intensity in Table VII. ; but further comment is unnecessary until the observations 
shall have been repeated when the vessel is aground, and her head in the directions 
noted above. 
The horizontal vibrations for intensity were obtained with Hansteen’s needle on 
Tarbert Island, but the state of the weather prevented me using that instrument on 
board. 
During the several revolutions of the vessel I availed myself of the opportunity to 
ascertain the deviation in different parts of her, so far at least as common compasses 
were capable of developing it, and the observations at P, L, G, Q, T, C must be 
scrutinized only with reference to the nature of these instruments*; for although 
good of their kind, they were too heavy to be influenced by those minute changes 
that would have been indicated by delicately constructed instruments. 
Several of these observations were simultaneous, the bell of the vessel being struck 
as the signal for observation when her head was steady upon the required point. 
A platform, which was so constructed as to project over the stern on a level with 
the tafrail, enabled me to observe the deflections of a compass in that position, which 
was at the distance of 13^ feet from any kind of iron; and a temporary stage or 
poop, which was also erected at my request, 8 feet above the deck, afforded me the 
means of ascertaining the deviation in several positions at that elevation, as the 
vessel was warped round. 
Some of these observations indicate that the deflections of the needle had reference 
rather to the nearer portions of iron than to the position and distance of the centre 
of the mass ; for instance, the difference of positions of E and D (at which were placed 
small needles without the incumbrance of cards) with respect to the probable centre 
of the mass, will not account for the difference of deviations observed. 
§ 3. Several distinct 'positions reviewed from the results obtained, with reference to 
placing a Steering-Compass. 
The observations at F and M on the platform over the stern show that no advan- 
tage would be gained by placing a compass there ; and this remark is applicable to 
several of the other positions tried, as will be readily seen by the results inserted in 
Tables I. and V. 
There were other positions, however, which held out better hopes, from the devia- 
tion observed there being less than that which had been ascertained to exist in some 
* The exact positions of these compasses are inserted in Table X. page 288 ; and their relative places will be 
clearly understood by referring to the longitudinal section of the vessel. 
