1820.] Mr. Barlo2v's Essay on Magnetic Attractions. 299 



towards the west, the dechnation of the needle becomes more 

 westerly, and it reaches its maximum about half-past one, p.m. 

 It then begins to move in a retrograde direction, and at sunset 

 reaches nearly the same point that it pointed to at sunrise. 

 The most complete sets of observations on this subject are those 

 of Col. Beaufoy, which have been continued for these three 

 years, and have been regularly published in the Annals of Phi- 

 losophy. Mr. Barlow's hypothesis to account for this daily 

 variation is, that the sun possesses a certain quantity of magnetic 

 influence, derived from ferruginous particles in its composition. 

 He shove's, by a satisfactory mduction, that all the phenomena 

 of the daily variation are consistent with this hypothesis, and 

 completely explained by it, provided we admit that the magnetic 

 energy is not propagated instantaneously, but moves at the rate 

 of about 20,0U0,000 of miles per hour. 



The 15th section of Mr. Barlow's work deserves, I think, the 

 peculiar attention of my scientific readers. I shall, therefore, 

 terminate this imperfect analysis of a highly interesting work by 

 inserting the whole of this concluding section. I do it the more 

 readily, because several of the notions which the author seems 

 to entertain coincide very closely with some opinions respecting 

 magnetism which I have entertained for some time, and which 

 indeed struck me on perusing the very passages in Biot's elabo- 

 rate work to which our author alludes. 



General Remarks relative to the Nature of Magnetic Action, consequent Corrections, fife. 



129. At present I liave hinted at no hypothesis explanatory of the law of action 

 ■which m€iy be conceived to have place between the iron and compass. AVe know 

 that, agreeably to the theory, (ir^t, 1 believe, advanced by fiilbert, but since 

 adopted and extended by Coulomb, Bint, and others, (he ball of iron which I have 

 employed in my experiments being placed in the neighbonrhood of the great ter- 

 restrial magnet, has itself acquired a certain portion of magnetic influence; its 

 upper part possessing the boreal, and the lower, the austral quality. Conse- 

 quently, according to this supposition, the end of the needle, which possesses the 

 same, or the opposite magnetism, to that iJiile of the ball which is exposed to its 

 action, or which has a predominance of action, w ill be repelled or attracted. 



1 have great apprehension in calling in question a theory sanctioned by the 

 approbation of so many eminent writers who have investigated this subject since 

 the time of its first promulgation by our ingenious countryman; and shall not, at 

 least in the present instance, attempt to do so: I shall only observe respecting it, 

 that whether it becorrect or not, it furnislies no clue to any pccc/icfl? mode of com- 

 puting the angles of deviation, as we have found Ihem to obtain in the experiments 

 reported in tiie preceding pages of this work. I do not assert that it is actually 

 inconsistent with the observed deductions, but merely, that it leads to such a com- 

 plicated analysis as to render it wholly nstless as a practical theory; and, if I 

 were (o add that I have some doubts of tin- accuracy of the deductions on which it 

 is founded, I should be sui)|)orted by the opinion of some other writers on the same 

 subject. Ur. Young, for example, speaking of this hypothesis, or rather perhaps 

 of others necessary to support it, observes: "This is obviously improbable, but 

 still the hypotheses are of great utility in .-issisting us to generalize and to retain in 

 memory a number of particular facts, which would otherwise be insulated.*" 

 This is all th.it I wish to have conceded to me in what I am about to advance; it 

 is, in fact, all that I intend ; namely, to have something on which the mind may be 

 fixed, in order to see, a priori, the probable consequence of this or that particular 

 or relative position of the ball and compass. 



• Dr. Young's Nat. Phil. vol. i. p. 683. 



