1820.] Mr. Barlow's Essay on Magnetic Attractions. 303 



ever, I conceive, impossible to delermUie this question satisfactorily, independent 

 of observations in diftVrent latitudes. Those mentioned by Captain Sabine are 

 unfortunately not sufficient for this purpose, as either value of 7n gives us an error 

 so nearly the same in quantity, being in one case as much in excess as it is in defect 

 in the other, that we cannot decide between tliem : moreover, the way in which 

 these deviations are reported, is so equivocal, that they can hardly be considered 

 as forming a sufficient test of the accuracy of any general law. It is, therefore, 

 to future observations in dillerent latitudes that we must look for the decision of 

 the above question ; and these may probably show that, independent of the correct 

 determination of the value of m, it may be found necessary to introduce another 

 indeterminate quantity, to represent the law of ititensity of magnetic action iu 

 different latitudes. 



138. Ihave already stated (Art. 9S) that the Admiralty had complied with my 

 request, viz. to be allowed to affix one of the apparatus described in Section XIII. 

 on board some one of his Majesty's ships, and the Owen Glendower had been men- 

 tioned for the purpose; but for certain reasons it was thought best to defer the 

 experiment for a more convenient opportunity. 



The Lords of the Admiralty have since submitted my proposition to the conside- 

 ration of the Board of Longitude, who have been pleased to sanction it with iheir 

 approbation ; and I am, therefore, in hopes that some new order v. ill ere long be 

 issued for the experiment being put fully in practice, at least as respects those 

 points which, in the present state of the question, cannot but be considered as 

 doubtful. 



139. Before I finally dismiss this subject, I beg once more to observe, that I 

 wish not the viens wliich have been advanced in tliis Section to be considered in 

 the liglit of an hypotliesis, but merely as a convenient mode of illustration. It is 

 not impossible but that the hypothesis of Coulomb may lead (by a reduction of the 

 several forces) to a single resultant, who^e direction, and the amount of the devia- 

 tions produced by it, may correspond, or be analogous to those which we have 

 found to obtain in the expeiiments reported in the preceding part of this workf 

 and should this be the case, it would be highly satisfactory, both by confirming the 

 accuracy of my practical observations, and ilie theoretical deductions of the author 

 referred to above. I may further observe, that it would be sufficient that this 

 agreement be found to have place when the mass of iron is great, and tl;e distance 

 considerable; this being the case in my experiments. For having had a particular 

 object in view, namely, to find some law or method of correcting the deviation of 

 the needle on shipboard, I carefully avoided embarrassing myself with anv inqui- 

 ries which did not bear immediately upon that question ; I employed, tiierefore, 

 only needles of tlie usual length, and endeavoured to work at as great a distance 

 from them as was consistent with the nature of my investigation : I am, therefore, 

 by no means disposed to advance, that with small pieces of iron, and with needles 

 cf all lengths, the same laws will still obtain : indeed I have already pointed out 

 (Art. 70) that williin certain limits or distances they must necessarily fail. 



If, therefore, when the mass of iron is great, and the distance at which it acts be 

 considerable, the laws which I have developed should be found to be the necessary 

 consequence of the hypotliesis to which we have alluded, the agreement will fur- 

 nish one of tlie best proofs that has yet been given of tlie accuracy of the deductions 

 upon which that hypothesis is founded, and I sliould hope without detracting in 

 any manner from the value of the experimental results detailed in the foregoing 

 pages of this work. 



140. The reader, however, will perceive that while I admit the possibility of the 

 hypothesis of Coulomb being consistent with my experimental deductions, I have 

 by no means expressed myself very positive that such will be the case: indeed, I 

 liave hinted, in more than one place, thai I have some doubts with regard to the 

 truth of that theorj', and on tliis point it may not be amiss to offer a few remarks. 



The leading features of the hypothesis in question are, that the earth is actually, 

 orat least operates as, a powerful nmgnet; that it has two principal poles, the one 

 in a high north latitude, and the other in south latitude ; and that, abstraiting from 

 certain inequalities, it gives a directive equality to magnetized bars, which accord- 

 ingly arrange themselves in any plane passing through those poles. 



Aloreover, the earth being, according to this supposition, a magnet, it is imagined 

 to have, like all otiier magnets, the power of imparting its own qualities to every 

 mass of iron in its vicinity ; and consequently, if a bar or ball of iron be suspended 

 near the earth, thelower half of it will possess a south pole, and the upper half a 



