NATURAL PHILOSOPHY. 123 



a general review of the formulae employed and recommended, the 

 Ifeport states that 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 devi- 

 ations of the compass are represented by the formulae derived from 

 Poisson's theory with a correctness which is within the limits of error 

 of observation. It was also stated that the following conclusions 

 might be accepted as established : 



1. That the magnetism of iron ships is distributed according to 

 precise and well-determined laws. 2. That a definite magnetic char- 

 acter is impressed on every iron ship while in the building slip, which 

 is never afterwards entirely lost. 3. That a considerable reduction 

 takes place in the magnetism of an iron ship on first changing her 

 position after launching, but afterwards that any permanent change 

 in its direction or amount is a slow and gradual process. 4. That the 

 original magnetism of an iron ship is constantly subject to small fluc- 

 tuations, from change of position, arising from new magnetic induc- 

 tions. 5. That the compass errors occasioned by the more permanent 

 part of a ship's magnetism may be successfully compensated ; and 

 that this compensation equalizes the directive power of the compass- 

 needle on the several courses on which a ship may be placed. On 

 the effect of heeling, a considerable body of evidence is collected ; 

 the most important practical result, as to the amount of the heeling 

 error, is the great amount to which it reaches in certain ships and in 

 certain positions in the ship, several examples of even two degrees 

 of change for one degree of heel being recorded. The Report points 

 to certain desiderata: 1. That in the construction of iron vessels 

 regard should be had to the providing a proper place for the compass, 

 the present difficulty being to reconcile this with the requirements of 

 construction and of working the vessel. 2. That for throwino- lio-ht 



^5 ^3 



on the points which are still obscure, the complete magnetic history 

 of some iron vessels in various latitudes should be known : this might 

 be accomplished by observations of durations, and horizontal and 

 vertical force, made at various fixed positions, in some new iron ves- 

 sel, in an extended voyage in both hemispheres, and in which the 

 magnetical observations would be made an object of importance. 



In a lecture on the above subject by Mr. J. T. Towson, Secretary 

 of the Liverpool Marine Board, Mr. T. stated that in treating of the de- 

 viation of the compass in an iron ship, they must consider that it was 

 affected by magnetism permanent or sub-permanent and inductive : 

 the one was a magnet in all positions ; the other w r as a magnet only 

 when in a certain position with regard to another magnet. The per- 

 manent was the most considerable of the disturbing forces, and it was 

 an extraordinary fact that a great deal of the disturbing force de- 

 pended on the direction in which the head of the ship was placed at 

 the time of her building. The steamer Great Britain had been knocked 

 about in Dundrum Bay for a whole winter ; she had been in both 

 hemispheres ; she had been repaired in a dock with her head in a 

 contrary direction to that in which it was at the time of her building ; 

 still it was easy to ascertain on examination which way her head 

 pointed at the time she was being built. This was an instance of a 

 vessel retaining her magnetism, notwithstanding the causes he had 



