SOME DISCUSSIONS OF THE OCEAN MAGNETIC WORK. 



1905-1916. 



Bt L. a. Bauer and \V. J. Peters. 



CORRECTIONS OF MAGNETIC CHARTS. 



From time to time, attention has been called to the corrections required for the various 

 magnetic charts to have them conform to the values of the magnetic elements observed on 

 the Galilee and the Carnegie} Reference to these corrections will also be found in various 

 sections of this volume. 



The corrections in the case of the magnetic-declination charts (mariners' charts of 

 lines of equal magnetic variation), for the ocean routes generally traveled, have been 

 usually below 2°, though at times exceeding this amount. Unfortunately, the corrections 

 are frequently of the same sign, or in the same direction, for long stretches at a time. The 

 navigational error is Ukely, therefore, to be cumulative when the mariner is dependent 

 largely on the correctness of his magnetic- variation charts; this is the case in time of 

 storm or fog, when it is not possible to control the ship's position by observations on the 

 Sun or stars. 



In certain parts of the Pacific and the Atlantic Oceans the chart corrections have 

 been about 4°, while in the Indian Ocean they reached 6°, and off the southwest coast of 

 Australia, from 12° to 16° (see p. 328). 



The corrections for the charts of the Unes of equal magnetic inclination have usually 

 been less than 5°, though amounting in certain regions to 9°. 



The corrections for the charts of the lines of equal horizontal intensity have been on 

 the order of 0.005 to 0.015 c. g. s., and have even reached .060 c. g. s. on the most southerly 

 cruises. In general, the corrections were found to be on the order of 2 to 10 per cent. 



Erroneous assimiptions as to amount and sign of secular changes have been found to 

 be partly, sometimes largely, responsible for the systematic chart-corrections. 



A brief summary of the dechnation corrections in the Pacific Ocean will be found in 

 the September 1915 issue of Terrestrial Magnetism and Atmospheric Electricity. 



A future volume will contain a detailed investigation concerning the amount and run 

 of the chart corrections. Let it suffice to state here that the chart corrections as found 

 may be attributed to a combination of the following causes: observational error caused, 

 for example, by the use of more or less imperfect instruments, or arising from some other 

 source; erroneous determination, or incomplete elimination of deviation-error produced by 

 the magnetic character of the vessel on which observations were made; erroneous secular- 

 change data as above explained; paucity of observations in a given region; and, finally, 

 local disturbances, near land and over shallow areas. What irregularities may be expected 

 in the isomagnetic lines over the ocean areas in general, is a question often raised, the 

 discussion of which must, at present, be deferred. 



A good idea of the extent of the chief magnetic data available to constructors of 

 magnetic charts, before the work of the Galilee and the Carnegie, may be obtained by 

 examining Plates 23, 24, and 25, showing the tracks of the chief vessels on which magnetic 

 observations were made at sea during the period 1839-1916. The legends on the three 

 respective plates will furnish all required explanations. Of course, no attempt has been 

 made to represent also the data, principally of magnetic dechnation, obtained by naval 

 and other vessels in the course of th eir cruises, or in connection with survey-work. The 



'For tables showing corrections of magnetic-declination charts, see Terr. Mag., v. 15, pp. 57-82, 129-144; v. 16, pp. 

 133-136; V. 17, pp. 31-32, 97-101. 141-144, 179-180; v. 18, pp. 63-64, 111-112, 161-162; v. 19, pp. 38, 126, 204, 234-235; v. 

 20, pp. 69-70, 'l04; v. 21, pp. 15-18. 109-116. 



