1865.] on the Magnetism of Ships. 301 



"(1) The great increase in the number of iron ships, as well as in the 

 amount of iron used in the construction of such ships. 



" (2) The losses of iron ships. 



" (3) The advances which have been made in, and the present state of, 

 the science of the deviation of the compass. 



" We may consider these separately. 



" I . It is believed that for some years the number of iron ships con- 

 structed has greatly exceeded that of wood-built ships, and this is particu- 

 larly the case as regards passenger steamers. In such vessels iron is now 

 used not only in the construction of the hull but in decks, deck-houses, 

 masts, rigging, and many other parts of the ship for which wood was till 

 recently used. The consequence has been a great increase in the amount 

 of the deviation of the compass, increased difficulty in finding a proper 

 place for the compass, and increased necessity for, and difficulty in, apply- 

 ing to the deviation either mechanical or tabular corrections. 



" 2. Many recent losses of iron steamers have taken place, in which it is 

 probable that compass-error has occasioned the loss. In most of these, how- 

 ever, from the want of any record of the magnetic state of the ship, of the 

 amount of original deviation, and of the mode of correction, and from the 

 investigations into the causes of the loss being conducted by persons not 

 instructed in the science, and who are necessarily incompetent either to 

 elicit the facts from which a judgment can be formed, or to form a judg- 

 ment on those facts which are elicited, no certain conclusion as to the 

 cause of loss can be arrived at. The investigations are, however, suffi- 

 cient to show the want of a better and more uniform system of compass- 

 correction in the mercantile marine, and of more knowledge of the subject 

 among masters and mates. 



" 3. Since the first introduction of iron ships it has been a recognized 

 fact that they cannot be safely navigated without the compass being as it 

 is termed ' adjusted,' t. e. without the deviations being corrected either 

 mechanically by magnets or by a table of errors ; but at first the correc- 

 tion of each ship was a separate and independent problem. Now the case 

 is different. The theory of the deviation, its causes, and its laws, are now 

 thoroughly understood and reduced to simple formulae, leaving the nume- 

 rical magnitude of a certain small number of quantities to be determined 

 by observation for each ship separately; and further, by recording, re- 

 ducing, and discussing the deviations which have been observed in the 

 ships of the Royal Navy of different classes, numerical results, as to the 

 values of these quantities in ships of each class, have been determined 

 which promise to be of the greatest use in facilitating the complete deter- 

 mination of the deviation and its correction, and in suggesting modes for 

 constructing iron ships, and in the selection of the position of the standard 

 compass. The science of magnetism, in its relation to navigation, is, in 

 fact, in a position in some degree analogous to that in which the science of 

 astronomy at one time was. The principles of the science have been 



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