542 



of " semicircular" deviation from 28 45' to 23 0', the direction of 

 the force changing from 37 to 32. 



At Southampton, in June 1860, or nearly eight months after the 

 experiments made at Holyhead, the force had further diminished 

 from 0*390 to 0'235, or by one-half, corresponding to a decrease in 

 the "semicircular" deviation from 23 O f to 13 30'; whilst the 

 direction of the force approached the fore-and-aft line 25, or from 

 32 to 7 ; the quadrantal deviation remaining nearly constant 

 [-j-4l] the whole time included in the various observations. 



The unvarying tendency of the direction of the ship's force in the 

 ' Great Eastern ' to assume a fore-and-aft line, supports the view that 

 time, with the vibrations and concussions due to sea service, leads to 

 a distribution of the magnetic lines, of the nature of a stable equili- 

 brium depending on the average of the inducing forces to which the 

 ship is exposed ; the respective sections of the hull having north and 

 south polarity, being separated by lines approximating more nearly 

 a horizontal plane and vertical axis through the body of the ship ; 

 instead of the inclined axis and equatorial plane of separation due to 

 the magnetic dip of the locality, and divergence from the magnetic 

 meridian, of the hull while building. 



The practical information resulting from the example of the c Great 

 Eastern ' is, that prior to a newly built iron ship being sent to sea, 

 her head during equipment should be secured in an opposite direction 

 to that in which she was built ; and that the magnetic lines should 

 be assisted to be " shaken down " by the vibrations of the machinery 

 in a short preparatory trip prior to the determination of her compass 

 errors, or their compensation ; but especially that in the early voy- 

 ages vigilant supervision should be exercised in the determination of 

 the compass disturbances. 



Another important point, generally neglected when compasses are 

 adjusted by the aid of magnets in a newly built iron ship, is rendered 

 manifest by the results of this investigation ; namely, the necessity 

 of the errors of the compass being determined and placed on record 

 prior to the adjustment. Without the knowledge to be derived from 

 these observations of the magnetic force of the ship, all future 

 changes of magnetism and consequent errors of the compass are 

 mere guesswork both to those who adjust, and those in charge of 

 the navigation of the ship. 



