Ross. — On the Deviations of Ships' Compasses. 11 



4tli. Stern compass, W. by N. 1 N. ) -p..™ -.^o r\nA/ c^ • j. 



Forward compkss, S. W. by S. l S. j ^^^^^^^^^^ ^3 071 , or ^ points. 



5th. Stern compass, AV.N.W. ) -r.-a? hqo ak/ n • . 



-P, -t o iTtT 1, c< ^ Dittei'ence, < o 45 , or 7 points. 



JB orwarcl compass, b. W. by b. J ' ' ^ 



The last-mentioned difference was noted during a dense fog, and differs 

 Ig- points from No. 3, taken when the stern compass indicated the same 

 course, Vvl., W.N.W. The observations were taken when the ship was 

 on an even keel. The use of a card of deviations in this case is 

 dangerous in the extreme, as the officer on watch not only has to apply the 

 correction to the forward, or standard compass, but, in giving the course to the 

 man at the wheel, has to apply a second correction, both corrections being 

 different on each course : on some courses + , on others — . The liability to 

 err in applying these corrections is therefore great. It is not to be wondered 

 at if the captain, after having, at a late hour of the evening, given a course in 

 order to clear a certain headland, should at daybreak find his ship three or four 

 miles nearer the land than he reckoned upon ; or that, having at night given a 

 course which he expected would keep his vessel running parallel to the land, he 

 should in the morning find himself out of sight of land altogether. After 

 considerable experience in the adjustment of the compasses of iron vessels, I 

 have no hesitation in saying that I believe that the local attraction in vessels 

 navigating in these latitudes can be easily and effectually neutralized, and 

 that, by this being done, the safety of passengers and property would be 

 secured, and the officers in charge relieved from a mental strain to which they 

 x)ught not to be subjected. Let us see what are the inferences drawn by 

 Professor Airy from the elaborate investigations made by that eminent 

 philosopher on this subject. He says .• — 



1st. At any place the deviation of the compass in any ship, whether wood- 

 built or iron-built, may be accurately represented as the effect of the 

 combination of two forces, one of which alone would produce a disturbance, 

 following the law of polar-magnet-deviation ; and the other alone would 

 produce a disturbance, following the law of quadrantal deviation. Tn 

 northern magnetic latitudes, the nature of the effect of the first will usually 

 be the same as if the boreal magnetism were towards the ship's head ; in 

 southern magnetic latitudes, it will be usually the same as if the austral 

 magnetism were towards the ship's head — the quadrantal deviation will be the 

 same in all magnetic latitudes — and whatever the magnitude of the earth's 

 directive force. These are the disturbances that are produced by transient 

 induced magnetism only. The polar magnet deviation will, however, be 

 affected in a greater or less degree if the iron which enters into the 

 composition of a ship possess independent polar magnetism, similar to that of 

 a magnetized steel bar, i.e., not depending on the terrestrial magnetism at the 



