340 
ME. EVAJs^S OX THE DEYIATIOX OF THE CO^klPASS 
Table IV. A selection from the wood-built paddle-wheel steam-vessels, embracing 
vessels of various dimensions. 
The selected examples of Tables II. and III. must be understood as adopted to represent 
the general types of their classes : the examples in these Tables, of deviations observ'ed 
on Foreign stations, are selected from the evident skill and care that has been bestowed 
on the observations, and from their perfect illustration of the general nature of the mag- 
netic change in wood-built steam-ships. 
Table V. The magnetic coefficients of the steam-ship Great Eastern, as observed in 
various parts of the vessel by myself, conjointly with Mr. Ruxdell, the Secretaiy to the 
Liverpool Compass Committee. 
The general tabular arrangement it is considered requires little fuiiher explanation 
than the titles of the various columns convey. The column “ Ship’s force to head " is 
the natural sine of the corresponding value of the coefficient B, and is in fact the pro- 
portion of the ship’s force to head, to the earth’s horizontal force. The column “ Ship's 
force to starboard ” is the natural sine of the corresponding value of the coefficient C, 
and is the proportion of the ship’s force to the starboard side, to the earth’s horizontal 
force. 
The resultant of B and C is the ship’s force, and is in its dhection 
and amount 
[\/B^-j-C^] the equivalent of the “ true starboard angle ” (measured ffiom the bow round 
by the starboard side to the bow again), or “ neutral position ; ” and of the “ modulus” of 
polar-magnet deviation in Mr. Aiey’s paper in the Philosophical Transactions for 1856. 
It will be observed that in the results obtained from De^iations observed on Foreign 
stations (noted in italics), the total force of the ship is given in two lines, the upper 
line being the proportion of the ship’s force to the horizontal force of the earth at the 
place, the lower line being the proportion of the ship’s force to the horizontal force in 
England, so as to allow a comparison to be made of the absolute force of the ship at 
various stations. 
For convenience, I'OOO has been adopted to represent the value of the earth's hori- 
zontal magnetic force at the several English ports where Her Majesty’s ships are usually 
swung, viz. Greenhithe, Sheerness, Portsmouth, and Plymouth : for other stations I have 
adopted those given by Mr. Airy in the paper of 1855 alluded to ; and for several places 
not included in his discussion, reference has been made to the chart of Absolute Mag- 
netic Intensity by General Sabine *, employing for the reduction to the horizontal com- 
ponent, the most recent determinations of the magnetic dip with Avhich I am acquainted. 
A comparative Table of the foregoing elements, Avith the horizontal force as giA en by 
Gauss in his ‘ Atlas des Erdmagnetismus,’ reduced to the same unit of measure, is 
appended, as being probably of use in any future inquiry or discussion. 
I now proceed to direct attention to various features exhibited in the several Tables, 
* Keith Johnsox’s ‘Physical Atlas,’ 2ud edition. 
