144 Royal Society : — 



pany's ship the ' Prince of Wales ' fov York Fort, where he arrived 

 on the 16th of August, and after completing the Magnetic Observa- 

 tions which he had been charged to make at that station, proceeded 

 on the 30th of the same month, by the canoe route, to join Mr. 

 Palliser, who had quitted England some days before him, and had 

 taken the route by the United States to Canada and the Red River 

 Settlement, and thence to Carlton House, where the whole party 

 would be assembled in the fall. 



The care which Lieut. Blakiston bestowed upon his determinations 

 at York Fort appears to have been commensurate with the theoreti- 

 cal importance which, before he quitted England, he was aware would 

 attach to the results. In submitting these to the Society, I must 

 solicit a continuance of the patience and indulgence so kindly given 

 to me on a recent occasion ; for the subject of Terrestrial Magnetism 

 is far less generally understood than I believe it deserves to be ; 

 and there is often an apparent complexity in the details, especially 

 to those who are not familiar with the subject, which requires time 

 to be occupied in their elucidation. I shall commence with showing 

 the confirmation whiqh Lieut. Blakiston's results give to the ap- 

 proximate accuracy of the value assigned in the 'Philosophical Trans- 

 actions' for 1846, for the absolute magnetic force at its principal 

 point of maximum in the northern hemisphere. 



Those who are conversant, either from personal recollection or as 

 a matter of history, with the opinions regarding the phenomena of 

 terrestrial magnetism entertained in the tirst quarter of the present 

 century, will scarcely need to be reminded how generally the belief 

 then prevailed, that'the magnetic dip and the intensity of the niag- 

 Tietic force at different points of the earth's surface might be repre- 

 sented with at least a sufficient approximation by mathematical for- 

 midse, obtained by supposing the magnetism of the earth to be con- 

 centrated into two magnetic poles, very near to each other and to 

 the earth's centre ; the supposition being also equivalent to that of 

 an infinite number of small magnets parallel to each other, distri- 

 buted equally throughout the earth's surface. According to this 

 supposition, the greatest intensity of the magnetic force in each of 

 the two hemispheres should be found at the points where the dip 

 should be 90°, and the intensity should vary in the proportion of 2 : 1 

 between places where the dip should be respectively 90° and 0°. 



In the Arctic Expeditions of 1818, 1819, and 1820, I had an op- 

 portunity of measuring the intensitj- of the magnetic force at several 

 stations in the immediate vicinity of the dip of 90° ; and in the years 

 1821 and 1822, of comparing these measures with others made at 

 several points of the coasts of Europe, Africa, and America, and at 

 islands in the Atlantic Ocean (which I visited for the purpose of 

 making observations with the pendulum), in dips which, including the 

 Arctic stations, varied from 0° to 88° 47'. Tiie residt of this compa- 

 rison was to place beyond a question the irreconcilability of the phe- 

 nomena with the supposition of a coincidence between the points of 

 90° of dip and of the maximum of force. For example, the mag- 

 netic force was found to be considerably greater at New York, where 

 the dip was not more than 73°, than at the stations in the Polar Sea 



