32 SECTION PHYSICS. 



that last condition having never been stated. The state of the case 

 shortly would be this : with regard to quadrantal corrections with an 

 Admiralty compass, that, that which would be perfect and quite satis- 

 factory, and quite useful practically, as long as the ship remains about 

 the British Channel, or even about the British Islands ; if she goes 

 away to Labrador, where the horizontal component of the magnetic 

 force of the earth is much smaller, the quadrantal error would be 

 found to be greatly over-corrected ; or if she went to the magnetic 

 equator, where the horizontal force is much greater, the quadrantal 

 error would be under-corrected. Thus, the most valuable quality of 

 the quadrantal error is vitiated by the greatness of the magnetic 

 moment of the compass card in the best of the compasses hitherto 

 used at sea. Notwithstanding the fallacy in this respect, I should 

 say that the quadrantal error, if it were not possible to get compass 

 cards in which the defect would not exist, still the quadrantal error 

 ought to be corrected ; but there would be considerable awkwardness, 

 and the possibility of being liable to error, by being obliged to vary the 

 distance of the correctors according to the different latitudes. Thus, 

 the octantal corrector would want to be brought nearer the compass 

 at the equator, and moved farther away in high north or south mag- 

 netic latitudes. But, if we can get over the defect, and have the 

 quadrantal error, as has always been stated hitherto in books and 

 papers on the subject, perfect in all latitudes, it would be a very great 

 advantage. It was on this account, and also to avoid the octantal 

 errors that I was allowed a good many years ago to attempt to 

 produce practical working compasses with very small needles, and I 

 have now succeeded in obtaining an instrument which works well, 

 as I may say, from actual experience at sea with needles, the largest 

 of which is 3! inches, and of which the magnetic moment is utterly 

 inadequate to produce in any sensible degree the effect I have alluded 

 to. This is the largest kind of compass card that I have as yet made, 

 and it is really large enough for all practical purposes. I do not 

 think that any sailor could say that these divisions are not big enough 

 for him to see them, and certainly no navigating officer can say that he 

 cannot see them when he is using a sextant every day, and reading 

 the scale off to minutes. With this compass, correctors cannot be brought 

 nearer than within about 6 inches from the centre of the compass. 



