ON THE THREE REPORTS OF THE LIVERPOOL COMPASS COMMITTEE. 93 
discovering a Correction for the Deviation of the Compass produced by the 
Tron of the Ship, by G. B. Airy, Esq., Astronomer Royal” (Phil. Trans. 
1839, p. 167). 
“ Discussion of the Observed Deviations of the Compass in several Ships, wood- 
built and iron-built, by G. B. Airy, Esq.” (Phil. Trans. 1856, p. 53). 
“ Practical Illustrations of the Necessity for Ascertaining the Deviations of the 
Compass, &c., by Capt. Edward J. Johnson, R.N., F.R.S., Superintendent 
of the Compass Department of the Royal Navy.” 1st edition, 1848; 2nd 
edition, 1852. 
‘« Magnetical Investigations by the Rev. W. Scoresby, D.D.” 2 vols. 1844-1852, 
Journal of a Voyage to Australia and round the World, for Magnetical Re- 
search, by the Rev. W. Scoresby, D.D.” Lond. 1859. 
*‘ First and Second Reports of the Liverpool Compass Committee to the Board 
of Trade, 1857.” 
“Third do., 1861.” 
* Reduction and Discussion of the Deviation of the Compass observed on board 
of all the Iron-built Ships, and a Selection of the Wood-built Steam-ships 
in Her Majesty’s Navy, and the Iron Steam-ship ‘ Great Eastern,’ by F. J. 
Evans, Master R.N., Superintendent of the Compass Department of H. M. 
Navy” (Phil. Trans. 1860, p. 337). 
The first and most important general result which is derived from all the 
observations recorded in these works, and from many more which have not 
been published, is, that the observed deviations are represented by the formulz 
derived from Poisson’s theory with a correctness which is within the limits of 
error of observation. 
In saying this, we are in some degree differing from a conclusion which the 
Reports of the Liverpool Compass Committee draw from observed deviations, 
viz. that there is a difference in the amount of the quadrantal deviation in 
different quadrants, depending either on some quality of the iron as regards 
its capacity for induction in different directions, or on the greater or less 
time occupied in moving the ship’s head over one or other of the quadrants. 
That some difference may, under certain circumstances, be caused by the latter 
cause we do not dispute, but we are not satisfied that it is appreciable in the 
ordinary process of swinging. On the contrary, we believe that, within very 
small limits of error, Poisson’s theory may be considered as exact for the 
ordinary process of swinging a ship. As regards more lengthened periods, 
particularly when the ship has been exposed to mechanical violence, the 
hypothesis no doubt ceases to be exact; but even then the most convenient 
mode of treating the subject is analogous to that which is familiar in physical 
astronomy and other mixed sciences, viz. to consider the theory as exact, but 
the coefficients derived from that theory as being themselves subject to 
changes to be derived from observations, and reduced or not, as the case may 
be, to law. 
Mr. Airy, in the first paper to which we have referred, describes very 
eareful observations made by him on board of two iron ships, the ‘ Rainbow’ 
‘iron-built steamer, and the ‘ Ironsides’ iron-built sailing-ship. In the first, 
observations were made at four stations: station 1, near the binnacle, 
13 feet 2 in. from the stern ; station 2, at a part in which a standard compass 
would probably be placed, being 31 feet 9 in. from the stern; station 3, 
48 feet 3 in. from the stern; station 4, 47 feet from the knight-heads, or 
1513 feet from the stern. Each compass was raised 4 feet from the deck, 
Tn the <Tronsides’ the compass was placed in the position of the binnacle 
compass. 
