456 Analyses of Boohs. [June, 



From the calculations in the fifth section, it appears, that, 

 according to the hypothesis, " the dip has not an uniform 

 increase, but is changing now more rapidly than it has ever 

 before done since magnetical observations have been made. Its 

 decrease during the last five years has been nearly half a degree; 

 and if our principles be correct," Mr. Barlow continues, " it 

 ought to decrease nearly the same during the next five years ; a 

 short time, therefore, will either confirm or refute the hypothesis 

 on which we have founded the preceding computations. Agree- 

 ably to which we ought to find in 



1828 the variation 24° 29' dip 69° 43' 

 1833 24 26 . . 69 21 



" The dip, therefore, is at present changing more rapidly than 

 the variation ; and it will continue to decrease with the latter 

 for about 260 years, when the longitude of the magnetic pole 

 will be 180°; the variation will, therefore, then be nothing, and 

 the dip only 56°, which will be its minimum ; they will then 

 both increase together for the next 260 years, when the needle 

 will have its greatest easterly variation, and will then again return 

 towards the north, the variation decreasing, but the dip still 

 increasing, for 165 years longer; viz. till about the year 2510, 

 when the magnetic pole will be again on the meridian of Lon- 

 don ; the variation will be zero, and the dip being then at its 

 maximum will amount to 77° 43'." 



Part III. — On Electromagnetism. 



This portion of Mr. Barlow's work consists of three sections, 

 the first being a sketch of the present state of the science, in 

 which, after mentioning the long known facts respecting the 

 magnetic agency of lightning, and the early experiments of 

 Ritter, he concisely describes the late researches and experi- 

 ments of Prof. Oersted, M. Ampere, M. Arago, Sir H. Davy, 

 and Mr. Faraday. The second section relates to the mathema- 

 tical laws of electromagnetism. " In the preceding parts of this 

 work," the author observes, " I have attempted to reduce the 



laws of induced magnetism to mathematical principles ; 



and as soon as I heard of M. Oersted's discovery, I was desirous 

 to establish, on similar principles, the law of electromagnetism ; 

 but it was some time before I was able to construct an apparatus 

 convenient for the purpose. Having, however, at length effected 

 this necessary preliminary to my satisfaction, I proceeded to 

 make the course of experiments, and to undertake the investiga- 

 tions which form the subject of the present section." 



" My first object was to repeat very carefully all the experi- 

 ments of M. Oersted, MM. Ampere and Arago, of Sir H. Davy, 

 and Mr. Faraday, with some others suggested by the results 

 thus obtained ; and having attentively considered all the pecu- 

 liarities of action thus developed, I was led to consider that all 

 vhe apparently anomalous effects produced on a magnetized 



