36 SECTION PHYSICS. 



found, arising from induction of the compass needle upon soft iion 

 correctors, amounting in some cases to 7 or 8, I should wish it to be 

 understood that the Admiralty compass department have long recog- 

 nized the difficulty of applying these correctors for the quadrantal 

 deviation, and to avoid that source of error, or indeed any source of 

 error that may arise, they have inculcated the practice, and I think it 

 has been so far a very wholesome practice, of constantly observing the 

 amount of error of Ships' compasses. Sir W. Thomson rightly gave it 

 as a theoretical principle that when once this quadrantal deviation 

 was corrected it remained permanent in all parts of the world. That 

 which he described as the natural history of the ships of the navy we 

 have found very useful, and we know from it the quadrantal deviation 

 of every ship, and we also know this deviation remains constant in 

 all parts of the world. By not correcting the quadrantal deviation 

 we are enabled to see, knowing its constancy, that the compass has 

 been properly attended to. In attending to its values as given by the 

 officers in charge of the navigation from time to time as they progress 

 over the globe there is a great check upon them. We have found it 

 very useful in practice never to allow an officer to suppose that his 

 compass is correct, or that it can be practically accurately corrected. 

 Therefore from day to day they make careful observations, and it is 

 something like a man with his watch, if he knows it is five minutes in 

 error, he simply allows for the difference, and in the same way the quad- 

 rantal deviation is allowed for instead of being corrected. With reference 

 to these beautiful compass cards which Sir W. Thomson has brought 

 before us, I think I can see where they are likely to be of far greater 

 use than in the ordinary navigation to which he wishes apparently to 

 confine them. They appear to me to be likely to be of use in 

 those great iron-clads which he so well described, where there are 

 several apparently abnormal conditions, but which are in fact obedient 

 to law. I think they might be introduced there, and probably we 

 should have less trouble in correcting our compasses with the very 

 small needle that he employs than with the ordinary compass. That 

 leads me to another point. I consider Sir W. Thomson has done 

 good in calling attention to the advantage of using short needles. 

 When long ships were introduced both into the navy and the merchant 

 service, the idea prevailed that you should have large compasses, but 



