541 



in her construction ? Attention is here directed to the ships ' Birken- 

 head ' and ' Royal Charter,' which from their well-known magnetic 

 coefficients may be regarded as the types respectively of "hard" 

 and "soft" iron constructed vessels, and from their consideration, 

 as also from a review of the general results, these conclusions are 

 derived : 



1 . That in an iron ship of ordinary dimensions, a standard com- 

 pass can be placed, the deviations of which will but little exceed those 

 obtaining in wood-built steam-ships ; and further, that on changes of 

 geographic position, however distant, these deviations will be within 

 smaller limits, and can be approximately predicted. 



2. A divergence from these conditions will arise when the inductive 

 magnetism of the hull or machinery predominates ; and it is inferred, 

 especially from the example of the ' Royal Charter/ that large qua- 

 drantal deviation and fluctuating sub-permanent magnetism (due to 

 hull alone) are co-existent, and give rise to conditions of compass 

 disturbance which are beyond prediction, and which have hitherto 

 baifled inquiry and given a complexion to theoretical deductions 

 varying as regarded from different points of view. 



In order to examine the change which the original magnetism of 

 an iron ship undergoes after launching, a series of compass observa- 

 tions were made in the steam-ship c Great Eastern' prior to her quit- 

 ting the River Thames in 1859, and subsequently at Portland, Holy- 

 head, and Southampton* at the three first-named places within 

 short periods of time of each other. 



The results, from an Admiralty Standard Compass placed in a 

 position the least subject to influence from local masses of iron, were 

 as follows : In the first five days, from Deptford to Portland, the 

 ship's force had diminished from 0*585 to 0'480 [the earth's force 

 = 1-000], or nearly one-fifth ; representing a decrease in the "semi- 

 circular" deviation from 35 50' to 28 45' ; the direction of the 

 force, or neutral points, approaching the fore-and-aft line by 10, 

 or changing from 47 on the starboard bow to 37. 



At the expiration of the next six weeks, the ship in the interim 

 having made the passage to Holyhead, the ship's force diminished 

 from 0'480 to 0*390, or about one-sixth, corresponding to a decrease 



* The observations at Southampton were made after the paper was communi- 

 cated to the Royal Society, and are introduced by way of supplement. 



