248 DETERMINATION OF THE ELEMENTS OF TERRESTRIAL 



in the values will be found, and the assumption of Gilbert of 

 the invariability of ' the variation ' will be found untenable. 

 In 1634 Henry Gillibrand made a determination of the declina- 

 tion at Deptt'ord and found it 4° 6' E, whereas Borough and 

 Norman had noted IV 15' E as its value in 1580. Ever since 

 the announcement in 1635 of this discovery, this so called 

 secular variation has been carefully studied, but the cause is 

 unknown and Gillibrand's words are still true, " it must all be 

 left to future times to discover." 



The changes in declination may best be studied by construct- 

 ing a diagram showing graphically the results of observations 

 at any one place. The curve shown in the figure '^' has been 

 drawn by Schott and represents the changes obser\ed at Paris. 



C. A. Schor.t's curve s^howing changes in magnetic declination observed at 

 Paris since 1540. 



When such curves are drawn for different places their 

 various forms impress us with the difficulty of discovering the 

 laws according to which this secular change takes place, and it 

 is evident that the changes in the other elements must be 

 studied at the same time. 



(1) U. S. Coast and Geodetic Survey, 1882. 



