46 BULLETIN OF THE 



(abstract.) 



In this paper it is proposed to give a brief account of the pres- 

 ent state of our knowledge respecting the secular change in the 

 direction of the magnetic needle, as observed within the limits 

 of the United States and at some adjacent stations — from the 

 earliest to the present time. 



The collection of the material and its discussion formed part 

 of the work of the U. S. Coast and Geodetic Surve}'; the results 

 have just been published in pamphlet form by the survey office. 

 This paper contains in fifty quarto pages, first, an explanation of 

 the secular motion, as compared with other motions to which the 

 direction of the needle is subject; second, an exposition of the 

 mathematical treatment for the representation of that motion ; 

 third, an extensive collection of results of about 525 observations 

 at 52 stations; fourth, tables of the results of the discussion, 

 comparison between observations and computations, and con- 

 cludes with a table of decennial values of the magnetic declination 

 from the earliest time of record to the pi'esent time. It is illus- 

 trated by a diagram and a chart, the former exhibiting the nature 

 of the curve which conforms to the secular change, the latter 

 illustrative of the positions of the line of no-declination at two 

 epochs and of the region where the needle appears to be at pres- 

 ent almost stationary; the annual change for 1880 is marked on 

 it in figures. 



The magnetic declination (commonly called the variation of 

 the compass) varies with respect to space and time. It is a 

 matter of observation that a magnet, when light and delicately 

 suspended, is seldom or never at rest, but is always shifting its 

 direction, or in a state of oscillation or of tremor, and may be in 

 a state of sudden changes. These angular motions have been 

 classified as regular or periodic, and as irregular variations ; it 

 is the first and largest of the periodic motions which claim our 

 special attention. To distinguish it from other regular oscilla- 

 tions, a few explanatory remarks touching the principal laws of 

 changes will suffice. The solar-diurnal variation consists in a 

 systematic movement of the magnet having for its period the 

 solar day. Its character is the same for the greater part of the 

 northern hemisphere, viz. : about the time of sunrise the north 

 end of the needle is generally found approaching to or near its 

 most easterly deflection from the average magnetic meridian ; 

 this extreme position to the right is reached about 8 A. M., the 

 north end then begins to move to the westward and reaches its. 

 opposite extreme position about half-past 1 P. M. ; after this 

 epoch the needle gradually returns to the morning position, un- 

 dergoing more or less minor fluctuations. The range of motion 

 is greater in summer than in winter; it is greater in the higher 

 magnetic latitudes when the horizontal magnetic intensity is less 

 than in lower latitudes; it is also subject to an eleven year ine- 

 quality coinciding with the cycle of the sun spots — the greater 



