84 



should be made of those parts of the British possessions which were 

 adjacent to the position which observation had indicated as that of the 

 principal maximum of the magnetic force in that hemisphere. This 

 recommendation was carried out in 1843 and 1844, and the particulars 

 of the survey, together with the conclusions derived from it, form 

 No. VII. of the magnetic contributions in the ' Philosophical Trans- 

 actions' for 1846, Art. XVII. The geographical position of the maxi- 

 mum of magnetic force derived from the combination of the 78 sta- 

 tions of that survey was 52 19' N. and 91 59' W. of Greenwich, and 

 the absolute value of the force at its point of maximum was found to 

 be 14-21 in British units (i. e. of mass, a grain ; of time, a second; and 

 of space, a foot). As both the geographical position of the point of 

 maximum, and the absolute value of the force prevailing there, are sub- 

 ject to a secular variation, of which the nature, the period, and the 

 epochs are desiderata of the highest theoretical importance, and as 

 the determinations which are now made may therefore probably be 

 referred to as data by remote posterity, their confirmation, by the 

 observations of a second observer visiting the same localities within a 

 few years of the same date, furnished with different instruments, and 

 pursuing in some respects different methods, was viewed as a circum- 

 stance nfuch to be desired by the Committee of the Royal Society 

 appointed, at the request of Her Majesty's Government, to suggest 

 scientific desiderata, to be accomplished by Mr. Palliser's North 

 American Expedition. 



York Fort had been one of the stations visited by Lieut, (now 

 Lieut.-Col.) Lefroy, in the Survey of 1843-44. It is situated 

 nearly due north of the point of maximum deduced from that survey, 

 and less than 300 miles distant from it. The intensity of the force 

 at York Fort in July 1843, derived from the combined observations 

 of the inclination and of the horizontal force observed by Gauss's well- 

 known absolute method, was 14' 07 ; and by Mr. Fox's statical appa- 

 ratus, taking Toronto as a base, 14 '03. We have now to compare 

 with these Lieut. Blakistoii's results in August 1857, viz. 14-024 by 

 the combination of the inclination and the absolute horizontal force, 

 and 14*017 by a recent improvement of Dr. Lloyd's statical method, 

 which renders the result independent of changes which may take place 

 in the magnetic moment of the needle employed in the determination. 

 The first of these two last-named results has been computed by Mr. 



