24 



CHANGES OF THE MAGNETIC DECLINATION 



The results of the preceding tables have been thrown into curves, to which the 

 corresponding readings at Greenwich and Washington have been added/ These 

 readings have all been referred to the same scale, and thus present at a glance the 

 great difference in the magnitude of the diurnal motion as well as that of the dis- 

 turbances. The Greenwich observations were taken by means of photography ; the 

 Washington corresponding observations were also obtained by means of Brooke's 

 automatic photographic registration, and have as yet only been published in the 

 6th volume of the Astronomical Expedition to Chili, under the direction of Lieut. 

 Gilliss, U. S. N. ; Washington, D. C, 1856. 



For the Greenwich curves the zero line corresponds to 22° west declination. A 

 remarkable absence of disturbances of any magnitude as well as a small diurnal 

 range of motion at the time of the vernal equinox, is shown by the March curves 

 both for Van Rensselaer and Greenwich. 



There appear to be some disturbances common to both places, and if these 

 indications should not be accidental they are of an opposite character, that is, a 

 magnetic east deflection is presenting itself as a magnetic west deflection at the 

 other station, and vice versa. For this the reader may examine hours 17 and 5i 

 of the curve for January 18 and 19, hours from 6 to 8, April 20th, and one or 

 two other less striking cases. The needle at Van Rensselaer Harbor actually points 

 with its north end to the south of the astronomical west, and its meridional compo- 

 nent of the direction is pointing in a southern or opposite direction to the same 

 component at Greenwich or Washington. 



Absolute Declination. — The magnetic declination at Van Rensselaer Harbor was 

 determined on three occasions in the summer of 1854. Two difi'erent magnets 

 were used. 



Determination of June 9tb. Magnet A 68, mirror facing magnetic north. 



Mean reading on mark 338° 03' 



Astronomical bearing (N.) . .  . . . . . 93 29 E. 



Reading of meridian (N.) 244 34 



Magnetic meridian (N.) . . . . . . . 135 59 



Declination 108° 35' 



W. (of north) at l*" 52" P. M. local time. 



• See accompanying plates 1 and 2. , 



