Work in the Pacific Ocean 



605 



The Carnegie Institution work of the 

 past two years has already yielded suffi- 

 cient results to enable the United States 

 Hydrographic Office, with the aid of 

 these newly acquired data, to issue re- 

 cently a new chart of the "Lines of Equal 

 Magnetic Variation." 



It was found that in the North Pacific 

 Ocean previous charts were out 1° to 

 3° — amounts of sufficient importance to 

 safe and rapid navigation, especially as 

 it was found that the error was systematic 

 over large areas. For example, in the 

 region between San Francisco and Hono- 

 lulu recent charts gave systematically too 

 small a value of easterly variation (mag- 

 netic declination), so that the compass ac- 

 tually pointed 1° to 2° farther east than 

 shown by the charts used in directing the 

 course of a vessel between these ports. 

 Since the distance is about 2,000 miles, 

 and assuming an average systematic 

 error of but 1°, it might transpire during 

 a cloudy or foggy passage, when no sun 

 or stars would be visible and sole de- 

 pendence would have to be put upon the 

 compass and the log, that the vessel at 

 the end of her 2,000-mile voyage would 

 find herself too far north by about 1/60 

 of the distance traversed (roughly, 35 

 miles) — sufficient to prevent a successful 

 landfall ! 



It requires three sets of lines to com- 

 pletely map out the earth's magnetic lines 

 of force : 



1st. The Chart of the "Lines of Equal 

 Magnetic Declination," or, as the mariner 

 calls them, "Lines of Equal Magnetic 

 Variation." These lines connect the 

 places where the compass points the same 

 amount; for example, 5° east or west of 

 north, as the case may be. This chart is 

 the one of prime importance to the 

 mariner or to the surveyor who must 

 rely upon the compass. Unfortunately 

 but a comparatively few years suffice, on 

 account of the progressive changes oc- 

 curring in the earth's magnetism, to con- 

 siderably destroy the value of such a 

 chart. In certain parts — e. g., in the 

 vicinity of Rio Janeiro — six years are 

 sufficient to produce a change of 1° in 



the compass direction; on the average, 

 over the earth it requires about 20 years 

 to produce an alteration of 1°. Hence it 

 is necessary to reoccupy or repeat the 

 magnetic observations at a sufficient num- 

 ber of points over the globe to keep 

 "tab," so to speak, on these changes and 

 thus keep magnetic charts up to date. 



However, in terrestrial magnetism the 

 stage of determinism has not yet been 

 reached as it has in astronomy. Eclipses 

 and other astronomical events can be 

 predicted many years in advance with 

 unerring precision. Though knowing the 

 precise state of the earth's magnetism at 

 any given time, it is not yet possible to 

 predict what it will be but a compara- 

 tively short time later, with sufficient ac- 

 curacy and in every locality, for even 

 the purely practical purposes of life, to 

 say nothing of purely scientific demands. 

 While it may be that the problem of 

 magnetic predictions is a far more diffi- 

 cult one than that of astronomic events, 

 it certainly does not appear as compli- 

 cated as that of long-range weather fore- 

 casts. I believe that the prime reason 

 for the backward stage in terrestrial mag- 

 netism is to be ascribed to the general lack 

 of means hitherto available for the ac- 

 cumulation of the necessary data. We 

 are but beginning to appreciate that the 

 physical phenomena of the earth demand 

 equal attention with the study of celestial 

 phenomena, and that the solution of some 

 of the vexing questions pertaining to the 

 physics of the earth undoubtedly will be 

 accompanied with results of the highest 

 importance not only to science but to 

 man as well. 



Next, it is necessary for a complete 

 delineation of the earth's magnetic forces 

 to construct the "Chart of Lines of Equal 

 Magnetic Dip" and the "Chart of Lines of 

 Equal Magnetic Force." These two 

 charts, as far as the mariner's purposes 

 are concerned, do not require to be quite 

 as accurate as that of the "Lines of Equal 

 Magnetic Variation," since they are used 

 only in determining how much a com- 

 pass, disturbed by the iron in a modern 

 vessel, must be corrected. However, 



