668 MAGNETISM AND THE COMPASS. 



the Variation of all places lying in North Magnetic Latitude, and which, 

 in the case of London, has amounted to about 28^° in 315 years. The 

 Dip, on the other hand, has changed but little, or 4^'' in the same period ; 

 this is obvious, because the two magnetic axes, while they change their 

 Terrestrial Longitude in a considerable degree, do not vary much in 

 Latitude, and, consequently, will not greatly affect the Dip in places at 

 some distance from them. 



The points then, interesting to the navigator, are, first, the influence of 

 the earth's Magnetism upon his Compasses, and the influence the ship and 

 her iron has upon them in neutralising or modifying the first ; the second, 

 namely, the Local Deviation, is a subject which has become magnified 

 into vast and vital importance since the introduction of iron and steel into 

 the fabric of ships. 



The North Atlantic Ocean has this peculiarity, in a magnetic sense, 

 that it has over nearly all its area but one kind of Magnetism — Westerly 

 Declination (or Variation) and Northerly Inclination (or Dip). The 

 Magnetic Equator, or the line of No Dip, passes, as before said, obliquely 

 across the Terrestrial Equator, from Africa to Brasil. The line of No 

 Variation passes from Charleston, across the West Indies, to the River 

 Essequibo. 



Now, as will be seen presently, as the force of the ship's Magnetism 

 diminishes, so does that of the earth increase in its influence on the 

 Compass, so that a ship's corrected Compass does not hold the same 

 relative position with respect to both, with the same correction, into a 

 different Magnetic Latitude. Thus, a vessel with a large original error 

 in England will find that error reduced, perhaps to one-half, on reaching 

 the Equator ; and if sailing Northwards a very different relation will be 

 found. 



This is especially to be noticed on sailing into the Gulf of St. Lawrence, 

 or anywhere in a N.W. direction towards America. Frequently a ship 

 may be standing on one course to the southward of West across the 

 Atlantic, and then suddenly bears up to N.W.; and then, passing rapidly 

 towards the Magnetic Pole, the relation between the earth's Magnetism 

 and that of the ship upon her Compass undergoes a rapid and important 

 change, which, unattended to, has doubtless been the cause of many em- 

 barrassments and accidents. 



The Local Deviation of the Compass is its variation from the Magnetic 

 Meridian, which may be caused, as already noticed, by a peculiar state 

 of the atmosphere, Aurora Borealis, lightning, or the local attraction of 

 the ship, iron, &c. This is a subject of inquiry, which was first explained 

 by Captain Flinders, in the description of his surveys of the Australian 

 coast. To this subject his attention was directed, not only by some 

 anomalous differences which he found in the Compasses he used, but 

 by others recorded by Mr. Wales, who had accompanied Captain Cook ia 

 the capacity of astronomer, and his deductions, referring as they do to 

 wooden vessels, are still held to be correct. 



But the problem of Local Deviation in iron ships is a widely difTerent 

 one, and includes a large range of phenomena and considerations which, 

 for many years after the introduction of iron as a material for building 



