August 



3- i! 



•] 



SCIENCi:. 



51 



struments, and at the same time capable of exact determinations of 

 tlie intensity in absolute measure ; he also procured dip circles, and 

 availing himself of the additional temporary aid of Dr. Locke of 

 Cincinnati and of Professor Renwick of Columbia College, the ob- 

 servational work was fairly started, and has since been prosecuted 

 uninterruptedly by various assistants of the Survey. In conse- 

 quence of the dual or polar character of the magnetic force, it re- 

 sisted for a long time all attempts of measure expressible in the 

 usual units, but in 1S33 Gauss showed how this could be done, and 

 after the invention, in 1836, of the portable magnetometer his meth- 

 od for the absolute measure of magnetic intensity came into gen- 

 eral use. 



Those who are acquainted with magnetic observations, which 

 also include certain astronomical operations, know the delicacy 

 and refinement of its operations when great accuracy is demanded, 

 and it should also be remembered that in those early days of the 

 •development of practical methods there were none of those facili- 

 ties we now possess in the number of trained observers, in the 

 home manufacture of instruments, and in the many treatises for 

 instruction now at hand. Apace with the field work the office 

 work of computation and discussion was prosecuted, and the re- 

 sults were published from lime to time in the annual reports. 



At first the observations were confined to the vicinity of the sea- 

 coast, but it was soon found that the charts could not satisfactorily 

 be supplied with the values for the variation of the compass unless 

 the observations were extended sufficiently inland to give proper 

 •direction to the magnetic lines, or isogenics as they are called, 

 which curves determine the angular difference between the astro- 

 nomical and magnetic meridian for a certain epoch. IVIeanwhile, 

 surveyors from all parts of the country applied for information, not 

 only for the present value of the declination, but what was far more 

 difificult to answer, for ihe value at some earlier period. To satisfy ^ 

 this inquiry, and to provide for a better knowledge of the annual 

 change of the declination needed for the charts, a more systematic 

 general collection of all magnetic observations taken within the 

 limits of the United States from the earliest to the present time 

 was undertaken, and has been kept up since 1878, at which time 

 the field of activity of the Survey was enlarged by the change of 

 ■Coast into Coast and Geodetic Survey. This collection arranged 

 by States and Territories now comprises several thousand observa- 

 tions of declination, dip, and relative and total intensities, and to- 

 gether with the direct survey work, which in July last comprised 

 731 stations (many of them occupied several times at definite in- 

 tervals), constitutes the material from which most of the deduc- 

 tions were derived, and which mark the advancement of our knowl- 

 edge in this department of research contributed by the Survey. 



The first permanent magnetic observatories in North America 

 were established at Toronto by the British Government about 1S40 

 (which observatory is still in operation under the auspices of the 

 Canadian Government), and about the same time at Philadelphia, 

 1840-45, at Girard College. The latter was directed by Professor 

 Bache. who, after taking charge of the Coast Survey, took advan- 

 tage of the newly invented application of photography to automatic 

 registration, procured one of Brooke's magnetographs, and caused 

 it to be set up and to record continuously the variations of the dec- 

 lination and of the horizontal and vertical intensities at Key West, 

 Fla., between i860 and 1S66. After the lapse of half a sun spot 

 cycle, which is the minimum duration for which it is profitable to 

 keep up continuous observation at any one place, the instruments 

 were transported to Madison, Wis., where between 1876 and iSSo 

 a second series of observations was procured. When the support 

 and co-operation of the Survey was asked for the two international 

 •expeditions fitted out by the United States for polar research, the 

 needed magnetic instruments, both absolute and differential, so far 

 as the Survey could supply them, were furnished and the observers 

 were trained during the short time permitting. 



The magnetic records of the second year (1882-83) at Point 

 Barrow, Alaska, by the party in charge of Lieutenant Ray, were 

 made by the Brooke magnetometers, which had in the mean time 

 been altered for direct or eye-observations. 



A superior self-recording magnetic apparatus, known as the Adie 

 niagnetograph, after the Kew pattern, and likewise working by 

 means of photography, arrived here during the late war, but for 



want of funds was not set up until 1882. This superior instrument 

 was located at Los Angeles, Cal., and continues to give excellent 

 results. It is Intended to terminate this series towards the begin- 

 ning of the next year, and then remount the instrument, either in 

 Washington Territory near Puget Sound, or In southern Texas, in 

 order to cover as much as possible of the space for which, hereto- 

 fore, our knowledge of tlie laws of terrestrial magnetism was most 

 incomplete. 



The first isogonic chart published by the Survey, entitled ' Lines 

 of Equal Magnetic Declination," will be found in the annual report 

 for 1855, the last one in three sheets appeared In the annual report 

 for 1S82 : a comparison between these charts will show In the most 

 conspicuous manner the progress made in our knowledge In this 

 direction during the Interval. The index to scientific papers In the 

 annual report for 1881 under the heading of 'Terrestrial Magnetism,' 

 enumerates no less than sixty-six titles up to the close of 1880; this 

 will give some idea of the activity of the Survey in this department. 

 Several important investlgatations have appeared in the later annual 

 reports ; in the report for 1882 we have an appendix discussing the 

 distribution of the magnetic declination in the United States for the 

 year 1885; the results are based on observations at more than 

 2,300 stations. In the report for 1885 we have an Investigation of 

 the magnetic dip and intensity, with their secular variations and 

 their geographical distribution in the United States. This appen- 

 dix, 145 quarto pages, involved much labor for its preparation : It 

 is accompanied by three finely excuted charts, besides the Illustra- 

 tions in the text, and discusses no less than 2,000 dip observations 

 and more than 1,500 observations for intensity. The results for 

 secular change of dip and intensity are new. In the report for 

 1886 (not yet issued) we have in type the sixth edition of an inves- 

 tigation much sought after, namely, ' The Secular Variation of the 

 Magnetic Declination in the United States and at some Foreign 

 Stations." From a small beginning in 1859 this paper has grown 

 to be a complete depositor)' of magnetic results available for the 

 study of the secular change within our territory, and the author 

 discusses most thoroughly the laws governing this mysterious 

 movement, the cause of which is as yet entirely unknown, though 

 in its nature it must be cosmical, since we cannot think of any ade- 

 quate cause within the earth to produce, so far as we can judge, 

 with the utmost regularity, the observed angular motion of the 

 needle during centuries. 



The deductions rest on 1,071 observations made at ninety-four 

 stations. The earliest observations on our western coast date from 

 the sixteenth century (Sir Francis Drake), the earliest records on 

 the eastern coast dating from the beginning of the next century 

 (Hudson and Champlain). In this branch of research the Survey 

 profited by the use of the valuable collection of declinations and 

 dips, the earliest on record, made by Prof. E. Loomis (now of Yale 

 JZollege), who published them In Silliinan's Journal in 1838 and 

 1840, and without which our results would not possess the degree 

 of reliability they now have. In this sixth edition, which spreads 

 over 1 1 6 quarto pages, we have minute references to observations, to- 

 gether with their critical examination. The resulting secular change. 

 Illustrated by several diagrams, is expressed analytically and Is also 

 given in tabular form. The laws which so far appeared to govern 

 this motion are stated, and embrace the whole of the area of the 

 United States (inclusive of Alaska), and are given sufficient expan- 

 sion to facilitate their connection with similar relations referring to 

 Europe, South America, and eastern .Asia. 



The magnetic records brought home by the polar expeditions in 

 command of Lieutenants Ray and Greely were placed in care of 

 the Coast and Geodetic Survey : this material was subjected to 

 computation and (liscussion, and arranged for the press. The 

 Point Barrow work (1881-83) forms part \'I. of the olTicial publica- 

 tion of Lieutenant Ray's expedition (published in 1SS5), and the 

 work done at Fort Conger (1881-83) under Lieutenant Greely will 

 form .Appendix N'o. 139 of Vol. II. of the official publication now 

 passing through the press. 



The reduction, analysis, and discussion of the automatically reg- 

 istered material at the magnetic observatories still await sufficient 

 computing force to bring out the many laws and complex relations 

 due to the ceaseless changes of the magnetic force. 



The annual expenditure on account of terrestlal magnetism is 



