ON AN IRON STEAM-VESSEL. 
275 
§ 2. Results of Experiments. 
The principal results are arranged in a tabular form, with plans of the Garryowen 
(iron steam-vessel), so as to present at one view the relative positions of the com- 
passes in different parts of the vessel ; and the results obtained (both as relates to 
the horizontal deflections of the magnetic needle, and also as respects the dip and 
intensity) on board, compared with those observed with the same instruments on 
Tarbert Island. And I must here express my thanks to Professor Christie for 
his suggestions on observations to be made on the dip and intensity at two positions 
on board, the one near the head, and the other near the stern (A and B), where the 
centre of the dipping-needle was to occupy as nearly as possible the same position 
as the pivot of the azimuth-compass, with which the horizontal deflections were ob- 
served. 
From having noticed a considerable embarrassment in the movement of the com- 
passes, to which my attention was more particularly directed, viz. those at A and B, 
I was not entirely unprepared for the discrepancies that appeared between the first 
and second series of observations. 
It is necessary to remark in this place, that during the second revolution of the 
vessel the fires were lighted and the steam was f up’; but in considering the dif- 
ferences that appear, both as relates to the deviation deduced from the simultaneous 
observations and that obtained by the bearings of the cone of the distant mountain 
(see Table II.), and likewise the differences, which are still greater, in the results 
obtained during the second revolution of the vessel, especially towards the north 
(see Table III.), I am not enabled to attribute such differences to the circumstance 
of the fires being lighted ; nor am I prepared to place them to the account of the 
sheering of the vessel, as they far exceed any limits that might reasonably be allowed 
for such a possible circumstance ; but the freedom of motion of the needles might 
possibly have been affected by the local influence, or rather influences, in the vessel, 
that is, by the proximity of certain masses of iron*, probably imbued with magnetic 
powers acting in various directions, and consequently weakening the directive power 
of the needles and embarrassing their movements, notwithstanding that the com- 
passes were at the several distances from these masses, which are noted in Table X., 
page 288. 
It was my intention to have put that part of Their Lordships’ orders relating to the 
trial of Professor Barlow’s correcting-plate in execution at the position A on board ; 
but the results obtained there did not hold out a probability of its successful appli- 
cation, the discrepancies already alluded to being such as to prevent “ the plane of 
no deviation” being accurately determined ; and consequently the true position for 
fixing the plate (according to the law determined by Professor Barlow) could not be 
* The iron davits and stanchions abaft, and the spindle of the wheel, are here particularly alluded to. 
2 n 2 
