670 MAGNETISM AND THE COMPASS. 



(3.) Magnetic Lines of the Inductive and Betentive Magnetism the same. — 

 Whilst the spontaneous influence of simple induction must be to develop 

 a transient magnetic condition, having in each individual case a Polar 

 axis and Equatorial plane conformable to those of the earth at the place 

 where the ship is built; so, also, the Ketentive Magnetism developed 

 during the building must have corresponding Polar direction and dis- 

 tribution. 



(4.) Liability of original Magnetic Distribution to Change. — The original 

 distribution of the Magnetism, or casting of the magnetic lines, must be 

 liable to change, after the launching, under any violent mechanical action 

 on the ship, when lying with her head in a new direction, or sailing i» 

 remote regions of the globe, having very different directions of the earth'a 

 Magnetic Force. 



(5.) Sympathy of the Compass with a ship's Magnetic Changes. — All 

 changes in a ship's magnetic condition must tend to produce disturbance 

 in the action of Compasses on or about the deck. And the effect must be, 

 in however minute or insensible quantity in some particular cases, to 

 change the amount of the original Deviations.* 



It was the argument on the " retentive " condition of the Magnetism of 

 the ship induced by her building, that led to the controversy between 

 Dr. Scoresby and Professor Airy, and caused the former undertaking, at 

 his advanced years, the voyage to Australia, from the effects of which he 

 may be said to have fallen a martyr to science. 



Dr. Scoresby, in his letter to the Underwriters Association of Liverpool, 

 in 1854, drew up the following propositions, in addition to those he laid 

 down in his later work of the " Voyage of the Boyal Charter:'' — 



1. That the original Magnetism and Compass Deviation are specially 

 Liable to change in new ships, when meeting with heavy weather on their 

 first going to sea. 



2. That a Change of Course, aftei long steering in one direction, is liable 

 to produce a Change in the Compass. 



3. That adjusted Compasses are specially liable to change in the direc- 

 tion of over-compensation, and may dangerously mislead the navigator. 



4. That a stroke of the sea may produce a sudden change in the Compass. 



5. That a stroke of lightning may change a ship's Magnetism and 

 Compass Deviation. 



6. That f ^Mt sun, shining partially on an iron ship, might change her 

 Magnetism. 



7. That permanent magnets applied for the adjustment of Compass 

 Deviations must, with rare exceptions, tend to aggravate the error in ships 

 going far into another hemisphere. 



8. That a compass aloft affords an easy, practical, and, if duly elevated 

 and prepared for, an effectual remedy for the ship's disturbing influence. f 



The experience gained by the voyage of the Boyal Charter showed that 

 the blows and strains she underwent in her voyage out to Australia 

 diminished, or, so to speak, shook out the inequalities in her Compasses, 



* "Journal of a Voyage for Magnetical Research," 1859, pages 71, 72. 



+ " The Compass ia Iron Ships, &c.," by the Rev. W. Scoresby, D.D., pages 67, &!*. 



