26 



VARIATION OF THE MAGNETIC DECLINATION. 



retrograde in regard to the advance of the secular change (to the westward) ; this 

 is in conformity with the law as given by Dr. Lloyd in the Dublin discussion, where 

 the motion of the magnet is to the westward at this period of the year, or the 

 reverse of the Philadelphia deflection, but the secular change is likewise reversed, 

 the west declination diminishing at Dublin (at the same time or more accurately 

 between 1840 and '43). 



For further comparison I give here the results deduced from seven years' obser- 

 vation at Toronto between the years 1845 and '51, a previous working up of a 

 three years' series (middle year 1846) not being deemed sufficiently distinctive in 

 its results. The secular change is here 2'.0 per annum, increasing westerly declina- 

 tion, whereas it was 4'.4 per annum at Philadelphia in 1843; as in the above 

 result + indicates west, east deflection. 



In regard to the amount of the inequality, the two stations agree remarkably 

 well, the range remaining slightly below 2' of arc. It has been supposed that this 

 range at the same station is increasing or diminishing as the secular change increases 

 or diminishes. 



It may further be remarked that the general mean resulting from the above 

 discussion at Philadelphia, viz., 560.4, is identical with the value given in Part I. 

 of the discussion, there deduced by an entirely different combination. The annual 

 effect of the secular change, -f- 4'.4, is likewise in very close conformity with the 

 value given in Part I., as found by a very different process. 



The monthly values of the annual variation may serve to give the corrections 

 to observed declinations in any month of the year needed to refer the same to the 

 mean declination of the year, and may also be used in the more refined discussion 

 of the secular change, in both cases, only, when the greatest accuracy is required. 



