346 
ME. EVA^S OX THE DETIATIOX OF THE C0:MPASS 
early days considered experimental, and built of the best material, had very small qua- 
drantal deviations, viz. 1° and 1° 6' respectively ; indeed the Astronomer Eoyal expe- 
rienced difficulty in tracing the effects of ten’estrial induction. Bearing these facts 
in view, and taking further into consideration that in building Government vessels 
stringent conditions of contract as to soundness of material are enforced, and strict 
supervision exercised, it may be fairly assumed that the amount given of 2° to 4° repre- 
sents the average condition of a well-built ship of the best or superior iron. 
On the other hand, can the inference be drawn that an iron ship with large qua- 
drantal deviation implies inferior material being used in her construction 1 
Two of the most disastrous and fatal shipwrecks on record have occurred to iron- 
built vessels, whose magnetic characters are perfectly known. H.M.S. Birkenhead, 
whose permanency of magnetism (Table I.) was especially constant, and quadrantal 
deviation small, — a short time prior to her loss, — may be considered as the type, 
magnetically, of a “ hard ” iron-built ship : the Eoyal Charter, with a quadi-antal devia- 
tion of 6°, and \yhose sub-permanent magnetism fluctuated more than any vessel with 
whose records I am acquainted, may be considered as the tj-pe, magnetically, of a 
“ soft ” iron-built ship. It will be recollected that in the wreck of these vessels under 
their varied circumstances, both parted amidships ; the Eoyal Charter so suddenly and 
rapidly, as to invite serious attention to various points of her construction. 
It is difficult to draw any comparative conclusions from these two cases, but they are 
placed in juxtaposition as grounds for further mquiry as to the connexion between the 
amount of quadrantal deviation and the nature of the iron of the ship ; and it vnl[ be 
remarkable and not less useful, if, in the prosecution of such inquiiy, it can be esta- 
blished that the relative qualities of the material used in Eon ship-building can be 
discriminated by the subtle agency of the ship’s compass-needle. 
On the Coefficients B and C in Iron-built Ships, or that part of the Compass-disturbance 
arismg from the permanent magnetism of hard” iron, and that induced by the eer- 
tical part of the earth's force on the soft” iron in the ship. 
On examination of Table I., it is at once observed that in the majority of examples 
therein given, a permanency of magnetism exists so little affected by changes of 
geographic position, as materially to confirm the riews entertained by the Astronomer 
Eoyal in his earliest discussion (1838), that the effect of transient induced magnetism 
in iron-built ships is very small comparatively. 
The absence of Eregular fluctuations, and the gradual diminution of the ship’s force, 
analogous to the gradual decrease of the quadrantal deriation, are notable features ; 
and apparently the general permanency is but little influenced by concussions from the 
sea, or from the repairs in dock, which necessarily must have occurred during the 
several years over which the observations extend. 
The gradual diminution of the ship’s magnetic force is shown in the follo'wing 
