I'Jl 



now composed of plate iron and angle iron to an extent calculated to 

 divert the compass from its proper direction to the furthest practicable 

 limits. 



Then, again, the limits of voyages. The world is now circumnavi- 

 gated as frequently as in those days the Atlantic was crossed; and, 

 consequently, the varied direction of the earth's inductive influence is 

 now felt to an extent not contemplated in the days of Barlow. When 

 steamers rose into existence, another principle became active, for which 

 Mr. Barlow was unprepared. His theory was founded on the hypo- 

 thesis that the iron of ships was inductive and symmetrically arranged. 

 If either of these conditions is unfulfilled, Mr. Barlow's plates cease to 

 correct the compass. I need not occupy your time in showing that the 

 soft and cast iron on shipboard is not symmetrically arranged. It is 

 impossible that the inductive magnetism of a horizontal plate can 

 compensate for a vertical piece of iron in all latitudes, since the induc- 

 tive magnetism of the former is maximum at the Equator, but that of 

 the latter at the Magnetic Poles. 



In modern times it has also been proved that the other condition has 

 not been fulfilled. The magnetism of ship's iron is not all inductive ; 

 on the contrary, in steam-vessels and in iron ships, from the hardness 

 occasioned by cold hammering and rolling of the metal, some portions 

 assume the character of permanent magnets. Indeed, the Astronomer 

 Royal, in investigating this subject, has come to the conclusion that the 

 disturbing force on the compass, in iron-built ships, consists of a very 

 large force of permanent magnetism, for the following reason : "If the 

 principal mass of iron be at the same level as the compass, the effect 

 of inductive magnetism will vanish absolutely. It will vanish, also, if 

 the masses of iron at the same vertical distance below the compass 

 be arranged in symmetrical opposition on its different sides.* From 

 these considerations, there is good reason to conclude that it is in all 

 cases extremely small ; and, therefore, that the correction made in 

 one latitude will be perfectly accurate in any other latitude." I must 

 confess that I am at a loss to perceive the force of the logic of the 

 Astronomer Royal. I admit his premises, but I cannot perceive the 

 correctness of his conclusions. In the first place, we know that the 

 principal portion of soft and cast iron is below the horizontal level of 

 the compass ; and, in the second place, a much greater amount pf iron 

 is generally placed between the compass and bows than betw.een it and 

 the stem, and thus, neither of the conditions on which the disturbance 

 vanishes is complied with. 



* The Astronomer lloyal here means, by employing the term symmetrical, when each 

 magnetic force is coanteracted by an equal force in a contrary direction. 



