Edwin. — On the Deviations of Shij^s' Compasses. 129 



he probably considers it to be the result of inattention on the part of the 

 helmsman. One of the greatest difficulties which the navigator of the present 

 day has to contend against arises from the magnetic changes which take place 

 in iron-built ships on every change of latitude, especially in places where the 

 dip of the needle or magnetic latitude varies rapidly. These changes affect 

 the compass in a proportionate amount, and in cases where no special care has 

 been observed in the selection of a place for the compass by which the ship is 

 navigated the changes in the deviation become a source of great anxiety to the 

 mariner. It appears to me that considerable advantages in this respect would 

 arise from the increased employment of steel-built ships. During the process 

 of building the common iron-built ship becomes highly magnetised by induc- 

 tion, but does not become a really permanent magnet — it has more the 

 proper-ties of what is termed sub-permanent magnetism. A vessel built of 

 steel must, however, become a permanent magnet during the process of 

 building, and I think there would be much less change in the magnetic 

 character of this vessel than in the one built of iron, because the changes 

 would be due to hard iron only, while in the iron ship it arises from both 

 hard and soft iron. In the case of the steel ship the change takes place in the 

 inverse ratio of the horizontal force, while in the vessel built of iron the 

 change arises from this, and is also for soft iron in the ratio of the dip. In this 

 colony there is a difference of about ten degrees of magnetic latitude between 

 Auckland and the Bluff, and as the soft iron is the part most affected by 

 change of latitude it is evident that if it is of considerable amount its effect 

 upon the compass will be marked. I will now suppose that the co-efficients 

 have been found for a vessel at the Bluff, and that A — — 1°, B == -f- 15°, C = — 6°, 

 D = -f- 3° 30^, E = — 0° 30', and that it has been ascertained that there are + 3° 

 to be allowed for vertical induction in soft iron, then at Auckland B will have 

 decreased to 4- 5° 42' and C to — 4° 30', and the deviations of the compass from 

 which the co-efficients were obtained will have decreased at North 1° 46', at 

 N.E., 5° 28', or half a point, and at East, 9° 18', or very^ nearly seven-eighths 

 of a point ; and if the vessel was steered to make an east course near Auck- 

 land, using the same deviation as at Bluff, the result would be that the vessel 

 would be directed nearly one point too much to the southward, which would, 

 in so short a distance as five miles, cause an error in the assumed position of 

 one mile, or twenty per cent, in the estimated distance. I have omitted all 

 notice of heeling error in this instance, which, as already shown, may be such 

 as to very seriously interfere with the navigation of the ship if guided by an 

 uncompensated compass. The Government steam vessel " Luna " being builb 

 of steel enables me to bring forward a case in point. The co-efficients of this 

 vessel have been ascertained from observations made in Auckland by Mr. 

 Stewart, C.E., whose ever careful work I am glad of an opportunity of 



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