Sir William Thomson on Perturbations of the Compass* 363 



to which the received empirical theories of galvanism and mag- 

 netism virtually have for their basis the hydrodynamical theory 

 of these forces. 



I have not attempted to treat in like manner the other expe- 

 riments the details of which are given in the same memoir, 

 because my analysis applies exclusively to a cylindrical magnet, 

 or coil, of the same transverse section throughout. The action 

 of a large bar-magnet, and that of an iron core in the form of a 

 bar within a coil, would require very different treatment, on ac- 

 count of the forms not being symmetrical with respect to an axis. 

 The experiments made with the edge and flat side of the large 

 bar-magnet turned towards the small magnet showed that the 

 action depended in some degree on the form presented to the 

 latter. I take occasion to mention incidentally that the great 

 accession of magnetic force which was observed to result from 

 putting a soft-iron core in the coil is attributable, according to 

 the hydrodynamical theory, to the transverse circular motions 

 about the rheophore constituting the coil, which give rise to just 

 such setherial streams as are proper for magnetizing. (See 

 < Principles of Physics/ pp. 619 & 620.) 



The arguments, and the evidence from numerical comparisons, 

 which I have now adduced, should, I think, suffice for coming 

 to the conclusion that galvanic and magnetic forces are modes of 

 pressure of the aether in steady motion. 



XLIX. On the Perturbations of the Compass produced by the 

 rolling of the Ship. By Sir "William Thomson, F.R.S.* 



THE "heeling-error/' which has been investigated by Airy 

 and Archibald Smith, is the deviation of the compass pro- 

 duced by a " steady heel J} (as a constant inclination of the ship 

 round a longitudinal axis, approximately horizontal, is called). 

 It depends on a horizontal component of the ship's magnetic 

 force, introduced by the inclination ; which, compounded with 

 the horizontal component existing when the ship is upright, 

 gives the altered horizontal component when the ship is inclined. 

 Regarding only the error of direction, and disregarding the 

 change of the intensity of the directing force, we may define the 

 heeling-error as the angle between the directions, for the ship 

 upright and for the ship inclined, of the resultant of the hori- 

 zontal magnetic forces of earth and ship at the position of the 

 compass. These suppositions would be rigorously realized with 

 the compass supported on a point in the ordinary manner, if the 



* Communicated by the Author, having been read in Section A. of the 

 British Association at Belfast (1874). 



