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VARIATION (continued)

Dr. Halley published, in the last century, a theory of the variations of the compass. In this work he supposes there are four magnetic poles in the earth, two of which are fixed and two moveable, by which he explains the different variation of the compass, at different times, in the same place. But it is impossible to apply exact calculations to so complicated an hypothesis.

M. Euler, son of the celebrated geometrician of that name, has however shewn, that two magnetic poles placed on the surface of the earth will sufficiently account for the singular figure assumed by the lines which pass through all the points of equal variation in the chart of Dr. Halley.

M. Euler first examines the case, wherein the two magnetic poles are diametrically opposite; 2d. he places them in the two opposite meridians, but at unequal distances from the poles of the world; 3d. he places them in the same meridian. Finally, he considers them situated in two different meridians. These four cases may become equally important, because, if it is determined that there are only two magnetic poles, and that these poles change their situations, it may some time hereafter be discovered that they pass through all the different positions.

Since the needle of the compass ought always to be in the plane which passes through the place of observation and the two magnetic poles, the problem is reduced to the discovery of the angle contained between this plane and the plane of the meridian. M. Euler, after having examined the different cases, finds, that they also express the earth's magnetism, represented in the chart publilhed by Mess. Mountaine and Dodson in 1744, particularly throughout Europe and North America, if the following principles are established.

Between the Artic pole and the magnetic pole 14

Between the Antarctic pole and the other magnetic pole 29 23'.

53 18' the angle at the north pole, formed by the meridians passing through the two magnetic poles.

250 the longitude of the meridian, which passes over the northern magnetic pole.

As the observations which have been collected with regard to the variation are, for the most part, loose and inaccurate, it is impossible to represent them all with precision and the great variations observed in the Indian ocean seem to require, fays M. Euler, that the three first quantities should be 14, 35, and 65 degrees. In the mean time, the general agreement is sufficiently satisfactory.

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© Derived from Thomas Cadell's new corrected edition, London: 1780, page 304, 2004
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