872 Mr. J. R Napier on Ships' Compasses. 



" Theoretically, in one point of view, all the head would have s polarity, 

 and all the stern N. But my impression is, that in such a case, the 

 equatorial line will be modified and brought within the vessel, so that the 

 upper works, both forward and aft, might possibly have s polarity, and 

 the keel forward and aft, north. Pray do try this. 



S 



'* But with such a modification I should expect that the line of no-attrac- 

 tion would run obliquely through the ship, something like the dotted 

 line e g, and so as, if not running out, to be highest ajl, and lowest 

 forward? The line of no attraction on the two sides (keel N or s mag- 

 netic) should be the same. 



" I have just heard of another case of compass change by a shock, in 



the case of a collision, where a change of about 2J points took place in 



the compassesj 



" Yours very faithfully, 



" W. SCORESBY." 



Description of the Curves of no Deviation on the S.S. '■'■Fiery Cross:'' — 



Fig. 1 represents the sheer plan of the S.S. " Fiery Cross " lying at the 

 inclination in which she was built, and with her head pointing south 32° 

 west. 



The curved lines drawn from the fore foot to the quarter, denote the 

 hei<Thts of the curves of no deviation, on either side, as indicated by a 

 small 2 1 inch compass needle, held with its centre 2 feet from the vessel's 

 side. The small circles denote points of actual observation, and are 

 those from which the curves are laid down. Thwartship, sections of the 

 vessel are also shown at the points a, b, c, and d, and the positions of 

 the centre of the needle, in respect to them, denoted by a small cross. 



The small needle being moved upwards or downwards from the above 

 indicated positions, experiences the rapidly varying resultant action of 

 the induced magnetism of the iron of the vessel, in virtue of which 

 arise large horizontal deviations in its direction. In the case of the 

 needle being carried round the inside of the bulwarks, at a height of 2 

 feet above the iron gunwale, the magnitude and character of these 

 deviations are exhibited by the curves drawn in figure 2. The line in 

 the sheer plan, denoting the height at which the small compass was 

 carried round the gunwale, has been repeated higher up, so as to form 

 the axis for the curves, upon which axis the observed deviations have 



