On Magnetic Observations transmitted from York Fort. 147 



experience which Lieut. Blakiston has had in canoe-travelling will 

 have prepared him to profit by the opportunities it may afford for 

 observation, and the route referred to is one of the ordinary canoe 

 routes of the Hudson's Bay Company: with this addition, the deter- 

 minations of geographical position and of the value of the magnetic 

 force at its point of maximum, may be expected to be amongst the 

 most perfect, as they will undoubtedly be amongst the most import- 

 ant data, in this great branch of Physical Geography. 



I proceed to notice Lieut. Blakiston' s observations upon the mag- 

 netic declination at York Fort, which, taken in conjunction with those 

 of the survey in 1843-4^1, tend to substantiate conclusions of no 

 less theoretical importance than those with which we have been 

 occupied regarding the magnetic force. It is well known to those 

 who are conversant with the phenomena of the secular chanye of the 

 declination, that during the whole of the last century, and for some time 

 after the commencement of the present century, the secular change 

 which took place in the position of the isogonic lines in the northern 

 parts of the North American continent, consisted in the progressive 

 translation of the lines from west to east. The line of no declination, 

 for example, to which, when Halley collected and coordinated the most 

 trustworthy observations previous to the publication of his INIagnetical 

 Map in 1/02, he assigned a position "about the meridian of the 

 middle of California" (Phil. Trans., No. 148), appears inHansteen's 

 'Mappa hydrographica sistens Declinationes magneticas Anni 1787' 

 (Erdmagnetismus, Atlas), at the latter epoch, as crossing Lake Supe- 

 rior, and proceeding from thence in a direction west of north, so as 

 to pass altogether to the west of any part of Hudson's Bay ; whilst 

 from well-assured observations of a still later date we know tliat soon 

 after the beginning of the present century, places situated on the 

 western shores of Hudson's Bay had east declination, showing that 

 the line of no declination had passed over and was now to the east 

 of them. Consistently with this general movement of the isogonic 

 lines from west to east, the declination at York Fort, which, accord- 

 ing to the observations of Capt. Middleton (Phil. Trans. 1726, 

 No. 393, and 1731, No. 418) was at least 19° West in 1725, had 

 diminished to about 5° West in 1787 (Hansteen, I. c). In Septem- 

 ber 1819 it was found by Sir John Franklin to be 6° East (Journey 

 to the Shores of the Polar Sea, 1819-22, p. 26), and by Lieut.- 

 Colonel Lefroy in 1843, 9° 2.5' East. Thus we perceive that in Httle 

 more than a century (from 172.5 to 1843) the declination at York 

 Fort had changed progressively, by the operation of secular change, 

 not less than 28°, always in the direction of westerly decreasing or 

 easterly increasing; (which is in effect the same as a movement of 

 translation of the isogonic lines from west to east). 



In 1841 the Toronto OI)servatory commenced its observations, 

 and although (from defective instrumental organization) the conclu- 

 sions in regard to the secular change of the declination were not at 

 first as j)recise as could be desired, they were sufficiently so to justify 

 a strong persuasion that some very notable change hud recently 

 taken place in the order of the phenomena, and to lead to the com- 

 mencement, in January 184.5, of a special series of monthly determi» 



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