294 



3. The intensity of the earth's magnetic force, as estimated in a 

 horizontal plane, and that of the dipping-needle, as referred to its 

 natural direction. 



The variation of the magnetic needle is the deviation of the di- 

 rection in which it rests from the astronomical meridian, or the angle 

 between the planes of the magnetic and astronomical meridians. 

 This angle has long been known to be in a constant state of change, 

 and its alterations have been observed not only from year to year, 

 but from day to day. It has been ascertained that, independent of 

 the gradually progressive change by which the magnetic meridian 

 shifts its direction through large arcs in long tunes, a daily oscillation 

 takes place, which, in these magnetic latitudes, is of small amount, 

 and can only be rendered prominent by neutralizing the principal 

 part of the earth's directive power, according to a method proposed 

 and practised by Mr. Barlow. In the high magnetic latitudes visited 

 by the Expedition, however, the horizontal directive force of the 

 earth is naturally so much weakened by the effect of the dip, as to 

 allow these oscillations to be observed with great distinctness, with- 

 out artificial aid, by merely suspending the needle by a silk fibre. 

 By this mode of observing, Captain Parry and Lieutenant Foster 

 have found the diurnal change of variation to be seldom less than 

 one degree, and sometimes to have amounted to five or even seven 

 degrees ; with this remarkable addition, that the changes in its 

 amount appeared to them to have obvious reference to the position 

 of the sun and, less distinctly, to that of the moon. They decline, 

 however, entering into any investigation of the laws of the influence 

 of these bodies, leaving them professedly to those who are theoreti- 

 cally conversant with these subjects. 



In casting our eyes down the table of variations, in which are 

 registered, hourly and frequently half-hourly, from the beginning of 

 December, 1824, to the end of May, 1825, the positions assumed by 

 two needles (whose constructions, &c. are minutely described), it is 

 impossible not to be struck with the unsteadiness of the needles. 

 They appear to have been in a perpetual state of fluctuation, ad- 

 vancing or receding alternately and by impulses, and in some in- 

 stances passing their mean positions from side to side as often as nine 

 times in the twenty-four hours. This irregular fluctuation is one of 

 the most remarkable features of this class of the observations. 



By an abstract of this table, in which the positions of the sun and 

 moon, the state of the weather, the aurora borealis, &c. are recorded, 

 it appears that the influence of the sun in increasing the diurnal oscil- 

 lation is much more marked than that of the moon ; and that, con- 

 trary to received opinion, the aurora borealis seemed to have had no 

 influence. The regular increase of the amplitude of the diurnal 

 oscillations with the advance of the sun to the north is very striking, 

 and not to be mistaken. 



A series of observations on the horizontal position of the needle 

 under a directive force, reduced by Mr. Barlow's method, by Lieut, 

 Foster, forms the subject of another part of this communication. 



