342 
m. EVA^y'S OX THE DEYIATIOX OF THE C0:MPASS 
Referring to the magnetic changes of the vjood-built vessels (screw or paddle) in the 
progress of their respective voyages to, and in the southern hemisphere, of which there 
are several good continuous examples, it is e\ident that the coefficients B and C are 
dependent in their change chiefly on the alteration of the magnetic dip. and that the 
horizontal intensity is a minor element of disturbance ; it may therefore be inferred as 
a general rule, that in steam machinery, permanent magnetism bears but a small propor- 
tion to inductive. 
This appears sufficiently from examples in Tables III. and IV. ; as also that, as in the 
case of wood-built sailing vessels (discussed by General Sabixe in the Philosophical 
Transactions for 1849*), the wood steam-ship’s magnetism, on rapid changes of geogiu- 
phic position, falls short of, or “ lags behind,” the amount theoretically due to changes 
of magnetic latitude. 
The wood screw-ships Plumper and Highflyer f are instructive examples of the 
“ lagging behind ” of their magnetism ; in the former case, on the ship’s arrival at 
Vancouver Island, the semicircular delation was much in aiTear of the value due to 
the magnetic dip, as compared with that originally obtained in England. After an 
interval of sixteen months, on the ship being reswung in the same localitv', which she 
had not quitted, the deviations exceeded the original amoimt observed in England, 
approximating to the value due to the increased magnetic dip at Vancouver Island. 
Similar features are exhibited in the Highflyer employed m the China seas. 
We have now to consider the magnetic nature of the hull of the ii’on vessel combined 
with the steam machinery ; and in stating here the fact, of the former being, in the ma- 
jority of the vessels investigated, highly permanent and but slightly inductive, in contra- 
distinction to the steam machinery, I am only anticipating what will be hereafter given 
in proof from examples in Table I. 
The position of the standard compass with reference to the steam machineiy in an 
iron vessel, has therefore an important bearing practically and theoretically ; for if it is 
placed in proximity to the funnel, which may be considered as the zero or measuring 
point for the machinery in general, the inductive magnetism of the machinery is super- 
added on the subpermanent magnetism of the hull, and complicates the question appa- 
rently beyond research. 
An instructive practical example of this condition is afforded in the case of the 
‘Vulcan’ iron ship: in this vessel the steam machinery is further aft than is usual in 
ships either of the Navy or of the mercantile marine; and it will be observed that the 
standard compass is only 27 feet distant from the funnel, and is elevated on the poop-deck 
nearly to the level of the top of the latter. Viewing the nature of the Vulcan’s hull 
under the conditions of permanent magnetism, the maximum delation, or ship’s force, 
* Contributions to Terrestrial Magnetism, Part IX. 
t Commanded respectively by Captains Geoege H. Eiohaeds and Chaeles F. Siiadwell, C.B. The 
labour and attention bestowed by these officers in the determination of tlie compass deviations of their ships, 
and the consequent value of the results to the practice of navigation, and the theory of a sliip’s magnetism, 
are worthy of especial record. 
