ON AN IRON STEAM-VESSEL. 
281 
Taking into consideration the situation of particular portions of the iron-work, and 
also the magnetic effect of the head and stern of the vessel, (which is described at 
page 282,) about one seventh the length of the vessel from the stern is an advanta- 
geous place on board the Garryowen. 
The elevation above the deck should be such as to remove the needle from the 
separate actions of particular portions of the iron-work, so that if possible the little 
irregularities might disappear, and the joint effects of all the iron in the vessel be 
resolvable into one force, the power of which might be discovered, and perhaps con- 
trolled. If that of simple iron, by Professor Barlow’s correcting- plate, and if that of 
the pole of a magnet, it might be useful to ascertain how far another magnet in a 
given position was capable of correcting the deflections. 
How near the deck may be approached with security has yet to be determined. 
At the position I, onboard the Garryowen, (which is 13^ feet above the deck,) the 
horizontal deflections are inconsiderable, and the dip and intensity, so far as the very 
unfavourable weather permitted them to be observed, warrant the belief that a nearer 
approach might be made. This, however, can only be proved by experiment ; and the 
remarks at page 276, and also at page 286, respecting the necessary experiments, are 
strictly applicable in this place. 
Had the weather permitted, the effect of Professor Barlow’s correcting-plate would 
have been tried at position I ; but with the continuance of rain and gales of wind 
there was no possibility of proceeding further with any chance of that accuracy which 
the nature of the service directed by Their Lordships demanded. 
§ 4. Observations on the magnetic effect of the Head and Stern of the Garryowen. 
From the very remarkable deflections observed at the positions F and L*, and sub- 
sequently at M, I determined to ascertain the difference of effect which the head and 
stern of the vessel might produce on the compass. For this purpose I placed several 
magnetic needles on the small quay at the south-west side of Tarbert Bay ; and among 
them was a dipping-needle, adjusted in the magnetic meridian, with the face of the 
instrument to the east, and indicating nearly the dip that I had previously observed 
on Tarbert Island. All the needles were placed out of the reciprocal influence of 
each other, and assumed their respective magnetic meridians. 
Having taken my station at one of the compasses, Mr. William Laird and the 
engineer observed the others. A line was then passed from the vessel to the quay, 
for the purpose of measuring the several distances ; and everything being in readi- 
ness, 1 directed the Garryowen to be warped from the south-east quarter, with her 
head towards the instruments on the quay. It is essential to remember that the un- 
marked ends (or those which pointed to the south) of all the needles were nearest to the 
vessel, and at the distance of 214 feet they retained their natural directions in their 
>idcccxxxvi. 
* See Tables I. and V. 
2 o 
