TERRESTRIAL MAGNETISM. 37 



111 order^ however, not to lose the precious opportunity offered 

 by the proposed expedition for placing on record the most 

 exact determinations in localities so remote from ordinary in- 

 tercourse by simultaneous observations, in correspondence with 

 those at fixed stations, the expedition ought to be supplied with 

 all the apparatus and requisites for such observations. 



The laws of magnetism, under any circumstances important 

 to a great maritime power, are every day acquiring additional 

 interest by reason of the introduction of iron vessels into navi- 

 gation. The practicability of employing these for long voyages 

 will of course entirely depend on that of obtaining a satisfac- 

 tory correction of the disturbing influence of the magnetism of 

 the vessel. This influence, as appears from the recent experi- 

 ments of the Astronomer Royal, Mr. Airy, on the Rainbow 

 steamer, of 200 horse power, is enormous, and such as to 

 render the loss of the ship, if hazarded at sea in a long 

 voyage and in bad weather, hardly so much a matter of risk 

 as of certainty. To get rid of this influence is a problem 

 of no ordinary difficulty — a difliculty enhanced by the fact 

 ascertained in the experiments alluded to, of its originating 

 in two distinct modifications of the magnetic power. The 

 difficulty has been overcome, however, by the eminent ma- 

 thematician just mentioned, and the Rainbow, which before 

 was brought round with much difficulty from Liverpool to 

 London, is now running with the same confidence in her 

 compasses on the part of her commander as would be the case 

 in an ordinary ship, the cause of error being, as would appear, 

 completely counteracted. It is understood that the success of 

 the Rainbow has not only caused several other iron vessels to 

 be built for the navigation of the British seas and the Baltic, 

 but that projects on a far more extended scale are entertained 

 for communication by iron steamers with the most distant 

 regions. 



This striking instance may serve, in reply to any objection 

 which may possibly arise to the proposed expedition, as having 

 for its chief and ostensible object the promotion of merely 

 theoretical research. That object, it is true, is to perfect a 

 theory, but it is a theory, pregnant, as we see, with practical ap- 

 plications of the utmost importance. The correction operated 

 by Mr. Airy's process is complete (as he has himself shown,) 

 only for that particular magnetic latitude for which the adjust- 

 ments are made, and those nearly adjacent ; and, although he 

 has pointed out the course of proceeding by which it may be 

 hereafter extended, yet the subject is one but just broached. 



