Naval Professional Papers Volume 13 (Paperback)

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. 1883 Excerpt: ...again in the evening eastward to the same position. The amount of this daily variation amounted, then, to about half a degree. Since this time the fact has been completely investigated by very refined means of observation, and the following general results arrived at: Tho north pole of the needle begins between 7 and 8 a.m. to move westward, and this movement continues until 1 p.m. About this time the needle becomes stationary, and soon begins to retrograde east, but with a slower motion than that of its previous advance. About 10 p.m. the needle is again stationary at the point from whence it started. A smaller second oscillation now ensues during the night: the north pole moves slowly west until 3 a.m. and then returns again as before. The action of the sun is undoubtedly the cause of this daily disturbance of the magnetic needle; we may hence expect it to vary in different latitudes both as to time and extent; we require, however, further observation for determining whether the daily variations have the same direction in points of westerly declination as in points of easterly. In the southern hemisphere the direction of the daily oscillation is reversed: the north end of the needle here advances eastward and returns westward. The annual periodical variation of the needle was discovered by Cassini in 1786, who found that the north pole, from the vernal equinox to the summer solstice, moved eastward, and again retrograded west during the next nine months. This last motion, however, he found to exceed the previous easterly deviation, and constituted the yearly secular change. The direction of the horizontal needle is in no degree affected by its energy as a magnet, whether possessing a strong or weak magnetic power; still its direction and all the laws of i...