Magnetic Survey of the United States 93 



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 1, 1900 — 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 11^4 degrees east in 15S0 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 181 2. At the pres- 

 ent time the needle points about i6}4 

 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 



