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National Geographic : 1902 Mar
Contents
OF THE UNITED STATES foreign countries, by various members of the Survey. With the advancing years the de mands for practical information from surveyors and mariners became so heavy that on July I, 1899, there was created a special division, known as the "'Division of Terrestrial Magnetism." The magnetic work has thus been made one of the fundamental divisions of the work of the Survey, and it is now possible to undertake seriously a mag netic survey of the United States and countries under its jurisdiction according to the methods in use in similar under takings abroad. Nearly every civilized country is at present either planning or has already carried out a detailed mag netic survey of its dominions. OBJECTS OF A MAGNETIC SURVEY A magnetic needle or compass does not point " true to the Pole," as the old saying would have it, and as was dis covered by Columbus on September 13, 1492, but instead makes an angle with the true north and south line, this angle being anything you please, according to the location of the place where the com pass is mounted. Thus, in the United States, in the extreme northeastern part of Maine, a compass points 21 degrees west of north, while in the northwestern part of the state of Washington it points 23 degrees east of north; hence a change of 44 degrees from one end of our coun try to the other. There are portions of the earth where the "north" end of the needle points due east or due west, and even for a place between the mag netic North Pole and the geographical North Pole due south. In view then of the fact of the use of the compass by the surveyor to locate land surveys, by the mariner to guide him in storm and night, over trackless seas, and by the traveler to pilot him in unfrequented regions of the earth, it becomes the first object of magnetic sur veys to determine the amount by which the compass direction differs from the true direction, and to publish the quanti ties in such a form so that those inter ested may, at a glance, be able to extract the desired information. The chart of lines of equal magnetic declination in the United States for 1900, based on over 4,000 determinations in different parts of the country, is a specimen of the form now generally adopted for giving this information in a convenient form. At the places along any one line, e. g., the line marked 8 degrees east, passing through about the central part of the United States, the needle everywhere points 8 degrees east. Along the line marked zero, passing near Columbus, Ohio, and Columbia, South Carolina, the compass direction coincides with the true direction and the needle is " true to the Pole," etc. Next, attention is called to the fact that such a chart can only apply to a certain year-thus our present chart is for January i, 19oo-namely, not only does the needle not generally point due north, as already shown, but the amount of the angle by which it departs there from is continually undergoing change, during the day, during the month, and from year to year. Thus, at London, for example, the needle changed its direc tion from i I degrees east in 1580 to 24 degrees 12 minutes west in 1812, a change of 35 degrees in 232 years. A street a mile long, laid out in London during the year 1580, in the direction of the compass at that time, would have its northerly terminus by seven-tenths of a mile too far east, according to the compass direction of 1812. At the pres ent time the needle points about i61 degrees west at London. In this country the rate of change in the compass direction is not as large as at London, but nevertheless of sufficient magnitude to seriously affect the mag netic bearings of boundary lines. Thus, at Baltimore the needle pointed in 1670 about 6 degrees and 6 minutes west; in 93 MAGNETIC SURVEY
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