814 



SCIENCE 



[N. S. Vol. XXVII. No. 699 



netic elements and, accordingly, stations 

 were reoeeupied at closer intervals in time, 

 as well as in distance, than had previously 

 been the custom. In consequence, certain 

 unexpected and complicated features of 

 the secular variation could be satisfactorily 

 delineated. It was found that the secular 

 variation had not, in general, progressed 

 in accordance with the predictions from 

 the empirical formula established ten 

 years ago. 



We may safely make the claim that 

 in no other country than ours have the 

 phenomena of the secular variation 

 received such careful investigation; the 

 data we possess, in consequence, for this 

 study are unrivaled. Unfortunately, in 

 most other countries, owing to the lack of 

 existing, permanent organizations, the ob- 

 sei'vations have had to be prosecuted more 

 or less intermittently. It is very much 

 hoped that in about ten or fifteen years 

 with the aid of the magnetic data now 

 being accumulated by the Carnegie Insti- 

 tution, it may be possible to extend the 

 investigation of the perplexing phenomena 

 of the secular variation over the globe with 

 the requisite completeness. 



It will, of course, not be possible within 

 the brief space of twenty minutes to go 

 into all the various details respecting the 

 magnetic survey of the United States and 

 the results of interest which are being 

 derived from an analytical discussion of 

 the observations. "We shall have to con- 

 tent ourselves with the consideration of 

 but one or two of the more important 

 phases of the work. 



You have before you one of the magnetic 

 maps constructed on the basis of the avail- 

 able material— the "Lines of Equal 

 Magnetic Declination and of Equal An- 

 nual Change for January 1, 1905" 

 (Slide 4). What impresses one most is, 

 doubtless, the manifold irregularities and 

 twists in the various lines. Increased 



knowledge teaches us that the more numer- 

 ous the observations the more devious be- 

 come the lines of equal values. When you 

 see perfectly regular or smoothly flowing 

 lines you may rest assured that they have 

 been either smoothed out or conventional- 

 ized or that they depend upon but very 

 few data. Instead of the irregularities 

 being the abnormal features, they are the 

 normal ones, and regularities are, in fact, 

 the abnormal features. The ocean mag- 

 netic work of the Carnegie Institution has 

 likewise shown that practically every land 

 mass exhibits irregularities in magnetic 

 distribution over that which would prevail 

 were the same region covered with deep 

 water. 



Similar magnetic maps to the one shown 

 have been constructed for the dip or in- 

 clination, the horizontal intensity, the 

 vertical intensity and the total magnetic 

 intensity, as well as certain other special 

 maps, e. g., the magnetic meridians or 

 paths traced out by following the direction 

 pointed by a compass needle, etc. It is 

 hoped that before very long they will pass 

 through the press. These new maps all 

 unite in bearing common testimony to the 

 irregularities in the distribution of the 

 magnetic forces in the United States. 



In conclusion, permit me to briefly 

 sketch the general scheme I am following 

 in the analytical discussion of the magnetic 

 conditions prevailing in the United States 

 as exhibited by these maps. Some of you 

 are familiar with the general analytical 

 treatment by spherical harmonics of the 

 earth's magnetic condition, at any stated 

 period, initiated by Gauss. You will 

 doubtless recall that he carried the ex- 

 pansion of the potential to terms of the 

 fourth degree, inclusive, thus involving 

 twenty-four coefficients to be determined 

 by a least square treatment of the available 

 magnetic data. The later analysts have 

 extended the expansion to terms of the 



