Magnetic Survey of the Ocean
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-311
2025-11-29
8
Since the course of a ship at sea is mainly dependent upon the readings of the compass needle, it is of especial importance that the magnetic dip, declination, and variation over the surface of the oceans should be accurately known. For the purpose of determining these values experimentally, the Department of Terrestrial Magnetism of the Carnegie Institution at Washington, D. C., built, in 1909, a remarkable sailing ship, the Carnegie. Almost no iron or other magnetic material was used in this ship and the little that was used was placed so far from the magnetic instruments that it had no disturbing effect upon them. Even the auxiliary gasoline engine is almost entirely of bronze. Because of this non-magnetic construction the Carnegie has been able to correct errors of from one to fifteen degrees in the magnetic charts used by mariners.
In five cruises nearly 200,000 nautical miles were covered, observations being taken every 100 or 150 miles. In October, 1919, the Carnegie started on her sixth voyage to extend over a period of more than two years, gathering data for the correction of magnetic variation charts.
Figure 1 shows the method of making observations; the most delicate instruments are permanently mounted and placed inside the dome. At the left of the dome is a gimbal stand, shown on a larger scale, with a sea dip circle mounted upon it, in Fig. 2.
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