ON AN IRON STEAM-VESSEL. 
269 
And at a position (No. 6) 11^ feet before the tafrail, and 5f feet above 
the deck, the deviation was (when the boats’ davits were out) . . 11° 40' E. 
Ditto (when the boats’ davits were swung in-board) 6° 20' E.* 
The preceding facts showing the influence of small portions of iron in the vicinity 
of the compass, may be worthy the attention of the practical navigator ; for it will be 
seen by the results obtained at a position not far from the binnacle, that the differ- 
ence of deviation amounted to no less than 5° 20' under the mere circumstance of 
swinging the quarter-boat’s davits in-board from their usual position where the boats 
are hoisted up, to that place in which they would be secured in stormy weather 
at sea. - 
Bearing in mind the practical question of placing a steering-compass as near to 
a convenient part of the deck as possible, and not considering the local attraction 
developed by the preceding observations at No. 5. and No. 6. excessive, I fixed on 
positions near to them for placing the azimuth compasses^, viz. A and B. 
The positions A and B were therefore selected with reference to the nearer por- 
tions of iron ; the nearest iron to the pivot of one position being 5 feet 9f inches, and 
that to the pivot of the other 6 feet, except the painted wire of the forecastle sky- 
light, which I afterwards discovered to be iron, and which was 4 feet 1 inch from 
the compass at B. 
The exact distances of particular masses of iron from each compass will be found 
in Table X., page 288 ; and the relative position of the several compasses, according 
to measurement, will be readily understood by referring to the letters in the longi- 
tudinal section of the vessel (Plate XXIII.). 
It is necessary to remark in this place, that simultaneous observations could not 
be made at all the positions on board, as the compasses would then have been too 
near to each other ; therefore in referring to the letters denoting the place of each 
instrument, it is to be understood that the observations were made at different times, 
except where otherwise expressed. It must also be understood that the deflections 
at C, F, P, T, O, Q were observed with common compasses. 
Mr. William Laird, Jun., who accompanied me to Ireland, kindly undertook to 
superintend the observations at Station X. ; and to him I am also indebted for very 
accurate plans of the Garry owen, and likewise for noting the time during the obser- 
vations for the magnetic intensity. 
Having instructed the engineer of the vessel so far in the use of the theodolite as 
to enable him to observe the horizontal angle which the position A on board sub- 
tended with the magnetic meridian as the vessel was warped round, he was likewise 
* Although the deviation was less when the davits were swung in-board, yet the directive power of the 
needle would be different, and it is the difference of deviation that is here to be remarked. 
t These compasses were made by Mr. Gilbert, their pedestals being so contrived by him that Professor 
Barlow’s correcting plate could be conveniently turned to the different points, and raised or depressed as oc- 
casions might require by means of brass tubes and screws. 
