Navigational Affairs

Navigational Affairs

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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1891 edition. Excerpt: ...or astronomical north and south line was then called, and is called by sailors to the present day, the "variation" of the needle. Gilbert erroneously explained the different magnetic variations in different places by magnetic action of hills and headlands, and was thus led to the false conclusion that there would be no variation at great distances from the land or in the central parts of a great continent. We now know that the variation of the needle depends in the main on the fact that the magnetic axis of the earth deviates about twenty degrees from the axis of rotation, and that the amounts of the variation in different parts of the world are somewhat nearly as they would be if the distribution of terrestrial magnetism were regular as in a uniformly magnetized terella, but with its axis thus oblique to the axis of rotation. If this were exactly the case, the directions indicated by the compass would lie along great circles passing through the two magnetic poles, and the angles at which these circles cut the geographical meridians would be the actual variations in different parts of the earth, and the magnetic equator would be a circle on the earth's surface midway between the magnetic poles, inclined to the astronomical equator at an angle of 200. But, in fact, there are irregularities of distribution, such as those adduced by Gilbert to account for variation; only we do not find them related to distributions of land and water, as he imagined. It is curious to find the idea of headlands attracting the compass still cropping up again and again two centuries after it was first suggested by Gilbert, and fifty or one hundred years after advances in knowledge of terrestrial magnetism had shown it to be erroneous. I find in an unpublished...show more

Product details

  • Paperback | 88 pages
  • 189 x 246 x 5mm | 172g
  • Rarebooksclub.com
  • Miami Fl, United States
  • English
  • black & white illustrations
  • 1236509501
  • 9781236509505