TERRESTRIAL MAGNETISM, ETC. 311 



netism, or of the magnetism induced in her by 

 the vertical component of the terrestrial magnetic 

 force changing with her geographical position. 

 The binnacle of my new compass contains 

 appliances, for making, with ease and certainty, 

 the proper changes in the adjustment of Airy's steel 

 magnets, whenever observation shows change to be 

 necessary. It has also an adjustable appliance for 

 placing properly a steel magnet below the centre 

 of the compass to correct the heeling error, accord- 

 ing to a subordinate but still very important part 

 of his complete method of correction. My 

 binnacle has also appliances for placing and fixing 

 once for all a pair of iron globes in proper 

 positions on the two sides of the compass to cor- 

 rect the quadrantal error. When the globes for 

 correcting the quadrantal error have been once 

 properly placed, no change of this adjustment is 

 ever necessary for the same ship, and the same 

 position of the compass in it, except in the case 

 of some change in the ship's iron, or iron cargo, or 

 ballast, sufficiently near the compass to sensibly 

 alter the quadrantal error. But the magnetic cor- 



