Epwiy.—On the Deviations of Ships’ 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 affeet - 
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 
properties 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= + 15°, C= — 6°, 
D= +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 + 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 built 
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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