Feb. 2 1, 1878] 



NA TURE 



333 



When the quadrantal error has been thus once accurately cor- 

 rected, ths correction is perfect to whatever part of the world 

 the ship may go, and requires no adjustment at any subsequent 

 time, except in the case of some change in the ship's iron, or of 

 iron cargo or ballast sufficiently near the compass to introduce a 

 sensible change in the quadrantal error. The vast simplification 

 of the deviations of the compass effected by a perfect correction 

 of this part of the whole error has not, as yet, been practically 

 appreciated, because, in point of fact, this correction had rarely, 

 if ever, in practice, been successfully made for all latitudes. 

 The pair of large needles of the compass ordinarily used in 

 merchant ships does not, as has been shown by Capt. Evans and 

 Archibald Smith, admit of the correction of the quadrantal error 

 in the usual manner, without the introduction of a still more 

 pernicious error, depending on the nearness of the ends of the 

 needles to the masses of chain, or of soft iron of whatever kind, 

 applied on the two sides of the compass to produce the correc- 

 tion. The Admiralty standard compass, with its four needles 

 proportioned and placed according to Archibald Smith's rule, is 

 comparatively free from this fault : but even with it, and still 

 more with the stronger magnets of the larger compasses of 

 merchant ships, thtre is another serious cause of failure depending 

 on the magnetism induced in the iron correctors by the compass 

 needles, in consequence of which, if the quadrantal error is 

 accurately corrected in one latitude, it will be found over-cor- 

 rected in high magnetic latitudes, and under-corrected in the 

 neighbourhood of the magnetic equator. The new compass was 

 specially designed to avoid both these causes of failure in the 

 correction of the quadrantal error ; and experiment has shown 

 that with it the correction by such moderate masses of iron as 

 those indicated in the preceding table, is practically perfect not 

 only in the place of adjustment, but in all latitudes. 



When once the quadrantal error has been accurately corrected, 

 the complete application of the Astronomer- Royal's principles 

 becomes easy and sure, if the binnacle is provided with proper 

 appliances for readjusting the magnetic correctors from time to 

 time, whether at sea or in port. But the system of nailing mag- 

 nets to the deck, in almost universal use in the merchant service, 

 is not satisfactory, and is often even dangerous. It always renders 

 needlessly tedious and cumbrous the process of readjustment by the 

 adjuster m port, and it leaves the captain of the ship practically 

 no other method of readjustment at sea than removing the mag- 

 nets altogether, or taking them out of their cases and replacing 

 them in inverted positions. The Astronomer-Royal himself 

 pointed out that his correcting magnets should be mounted in 

 such a manner that their distances from the compass can be 



gradually changed, so as always to balance the ship's magnetic 

 force as it alters, whether by gradual loss of her original mag- 

 netism through lapse of time, or by the inductive influence of 

 the earth's vertical magnetic force coming to zero, and then 

 becoming reversed in direction when a ship makes a voyage 

 from the northern to the southern hemisphere. The not carrying 

 out of this essential part of his plan, whether through no method 

 or no sufficiently convenient method of adjustment having been 

 hitherto provided, has undoubtedly taken away much of the 

 credit among many practical men to which the Astronomer- 

 Royal's method is justly entitled. I have, therefore, introduced 

 into the binnacles provided for my compass, when it is to be 

 used in iron ships, a complete system of adjustable correctors for 

 perfectly correcting the error of the compass for every position 

 of the ship's head when she is on even keel, and a vertical 

 adjustable magnet below the compass, for correcting the heeling 

 error (more properly speaking, a magnet, which is vertical when 

 the ship is on even keel, and which shares the inclination of ttie 

 ship wlien she heels over to either side). 



An objection which has often been made to the use of correctors 

 at all, and particularly to the use of correctors for a standard 

 compass, is that they conceal the actual state of the ship's 

 magnetism, and that readjustment of the correctors at sea leaves 

 the navigator without means of judging, when he returns from a 

 foreign voyage, how much of the changed error found on read- 

 justment in port depends on changes which have been made in 

 the correcting magnets, and how much on changes of the ship's 

 own magnetism. This objection I meet by providing that at 

 any moment my correctors can be removed or set to any degrees 

 of power to which they may have been set at any time in the 

 course of the voyage, and again reset to their last position with 

 perfect accuracy. The appliances for changing the adjustment 

 are under lock and key, so that they can never be altered, except 

 by the captain or some properly authorised officer. Farther, to 

 facilitate the use of the correctors, I graduate the scales accu- 

 rately to correspond to definite variations of the force which they 

 produce on the compass. Thus, as soon as the error has been 

 determined by the known method of observation at sea, the 

 corrector may at once be altered to the proper degree to correct 

 it. Of course the oflftcer performing the adjustment will satisfy 

 himself of its correctness by a second observation. The objection 

 of "delicacy of manipulation," and the difficulty of carrying it 

 out, except by a professional adjuster, of which so much has been 

 said, is entirely done away with when adjustable correctors, with 

 scales thus accurately graduated, are provided with the binnacle. 



The binnacles before you are of two kinds adapted to the two 



