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. 1870 Excerpt: ... .... 2--a--2-3 or 2-(l-3sin'J)= Q. These four quantities M, N, P, Q, do not depend on the terrestrial force or on the position of the ship, but are truly constants of the ship, depending only on its construction and its susceptibility of magnetism. Then the disturbing forces are, In x or towards the magnetic north, ...
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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. 1870 Excerpt: ... .... 2--a--2-3 or 2-(l-3sin'J)= Q. These four quantities M, N, P, Q, do not depend on the terrestrial force or on the position of the ship, but are truly constants of the ship, depending only on its construction and its susceptibility of magnetism. Then the disturbing forces are, In x or towards the magnetic north, -HM + HP. cos 2A+VN. cos A. In y or towards the magnetic east, + HP. sin 2A + VN. sin A. In z or vertically downwards, -VQ + HN. cos A. These are the forces which act on the red end of the compass-needle. Those which act on the blue end are of the same magnitude but opposite signs, and therefore merely double the power which produces deviation of the needle. 69. Examination of the physical meaning of the different terms of this disturbing force. First. If we compound together the terms VN. cos A towards the north, and VN. sin A towards the east, we find that they produce a term VN directed in the azimuth A, that is, directed to the head of the ship. This term therefore resembles in all respects a permanent magnetism of the ship, so long as the ship remains in one place. But it vanishes when V vanishes (that is, at the magnetic equator): and it changes sign when V changes sign (that is, in the south magnetic hemisphere): and this circumstance will give facility for determining the influence of this term, and correcting it by a magnet at each place of the ship. It will be seen from the expression for N that if the whole mass of iron is either at the same level as the compass (making sin b--0), or below the compass (making cos b = 0), the expression for VN vanishes. Second. The terms--HM+ HP. cos 2A shew that there is a term of fluctuating value in the meridional direction; if however P vanishes, that is if all the iron be below the compass, th...
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