IN THE lEON SHIPS OP THE EOYAL NAVY, 
341 
and shall venture to suggest what would appear from fair inferences to be the leading 
principles of the magnetism of the ships included therein, and their practical applica- 
tion. 
It has been deemed unnecessary to introduce an analysis of the compass deviations of 
wood-built sailing vessels ; as, whether as ships of war with batteries of guns, or merchant 
vessels carrying cargo, — always excepting iron, — their deviations are small in amount, 
and arising in the aggregate from induced magnetism. 
An intercomparison of the magnetic coefficients of iron and wood-built steam-ships 
is of greater importance in the investigations of the laws affecting the former, than 
would at ffi’st appear : the special points elicited are — 
That D, which in the iron vessel ranges in value from to +6° and 1° (the Liver- 
pool Compass Committee recording even a point or 11°), seldom exceeds +1° in wood 
screw-ships, with not unfrequently a small minus sign in these vessels, and +1-^° in the 
wood paddle-wheel steam-ships. 
A and E appear common in character and value in all the classes of vessels enume- 
rated. 
B and C thus differ : — In the wood-built vessel (in Great Britain) B is + whenever 
the engines are before the compass, and C is always small in value. In the iron-built 
vessel, B, irrespective of the machinery, is either + or — as the ship’s head while 
building was south or north ; and C may be large in value, either + or — ; B nearly 
vanishing as the ship’s head while building deviated from the magnetic meridian and 
approached the east or west points of the compass*. 
Taking therefore the relative tonnage, horse-power, and position of the standard com- 
pass in each class of vessel, wood and iron, a judgment can be formed of the comparative 
magnetic effects of the engines and boilers, apart from the hull or hammered fabric of 
the iron ship. 
Several examples occur in the Tables ; from which we may infer, (1) that in an iron 
vessel built in England, head the north end of the needle is drawn to the bow, or 
B is positive. The steam machinery has the same effect ; B therefore as due to the hull, 
is increased in amount. (2) In an iron vessel built head to the north, the north end of 
the needle is drawn to the stern, or B is negative ; the steam machinery will in this case 
tend to diminish the value of this coefficient ; and on reference to Table I. it will be 
observed that, as a rule, those vessels with B positive have, taking size and other con- 
ditions of compass position into consideration, large compass deviations. 
But what are the combined effects of steam machinery and hull on a great change of 
geographic position, or in high south magnetic latitudes'? An examination of the 
several Tables elicits some valuable information on this important point. 
* The connexion between the direction of the ship’s keel and head while building as referred to the mag- 
netic meridian, and the direction and strength of her magnetic polarity, so fully experimented on by the 
late Dr. Scoeesbt and the Liverpool Compass Committee, is, I consider, fully confirmed ; and there will be 
added to this Eeport the magnetic lines of the ‘ Great Eastern,’ as another example of this remarkable 
feature of an iron ship’s magnetism. 
