Ross.—On the Deviations of Ships Compasses. 11 
4th. hes compass, W. by N. N 
ward compass, S. W. by 8. 4 18. 
5th. Stern compass, W.N. W. 
orward compass, S. W. by 8. 
The last-mentioned difference was noted during a dense fog, and differs 
| Difference, 73? 071', or 61 points. 
| Difference, 78° 45’, or 7 points. 
li points from No. 3, taken when the stern compass indicated the same 
course, viz, 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 а 
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 
ought 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:— 
lst. 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. In 
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 tg that of 
а magnetized steel bar, ї.е., not depending on the terrestrial magnetism at the 
