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Art. V. An Essay on Magnetic Attractions, and the Laws
of Terrestrial and Electro-Magnetism. By Peter Barlow, Associate of the Society of Civil Engineers, and of the Royal Military Academy. Second Edition, much enlarged and improved." 8vo. 304 pp. Mawman. 1823.
We believe it is now considerably more than a year since we first offered to our readers some remarks on the magnetical researches of Mr. Barlow, as they then appeared in a small publication amounting to little more than a pamphlet. This gentleman, to whom both science and art, and we might add the nation at large, are under so great obligations, bas since that period pursued and extended his researches in the same and other kindred branches, with equal assiduity and success; and has now presented the public with a volume embracing a variety of important subjects, closely allied to his former investigations. Although the title of the work before as differs bat little from that of the former publication, (which is in fact here entirely republished, in many particulars extended and improved,) yet considerably more than two thirds of the volume are occupied by other new and important matter.
. Among the chief improvements upon the former essay, we must mention a highly interesting section giving an account of the trial of Mr. Barlow's method of correcting the local attraction on board His Majesty's ships Leven, Conway, and Barracouta. It will be needless for us at present to enter upon any detailed account of these operations : it will suffice to observe, that they appear to bave been attended with the most complete and decisive success. The testimonies given by the naval officers themselves are particularly strong in fayour of the method. A letter is inserted which Mr. Barlow received from Capt. Baldey on the return of the Leven from the coast of Africa, expressing his most unqualified approbation of the plan, and his conviction of its utility and efficacy. From this letter we cannot forbear making a short extract:
" You will perceive that in several instances our þinnacle com. passes differed from each other a half to three quarters of a point ; which however we were always enabled to correct by your plate, and in all cases our place buty reckoning, when thus corrected, agreed as closely with the observations as we could have reason to expect. Indeed little need be said to shew how very erroneous a place by reckoning must be found, after a run of several hours, five, six, or seven degrees out of the supposed course; At sea such an error, although very considerable, is not perhaps of much im. portance but in making land, in entering a channel, and in narrow seas, it might be, and doubtless has been, frequently attended with the most fatal consequences.
“ Under this impression, and being convinced from experience of the simplicity and efficacy of your experiments, I beg that you will make any use of this letter, which you think will be of the greatest service in bringing your method of correction into general practice." P. 106.
We conceive this testimony will speak for itself, and form the best practical comment on Mr. Barlow's researches, Another testimonial, even stronger than the last, is given by Lieat. Madge, in the case of the Barracouta; but of this our limits will not permit us to give the detail. We must refer those who may wish for further proof of the importance of Mr, Barlow's suggestion to the 13th section of this work; where we think they must admit its efficacy to be most satisfactorily and triumphantly established; and in which also we conceive practical men will find all the necessary information for enąbling them to adopt the plan, delivered in the plainest and, at the same time, in the fullest manner.
Mr. Barlow.has given a description of a model, which he has constructed, of a vessel, with all its iron properly distributed, ą small compass in its relative situation, and a correcting plate upon a proper scale; by means of which the effect of the local attraction, and the practical application of the means of cor recting it, are displayed in the most clear and satisfactory manner. This model be presented to the Society of Arts, &c.; and in consequence of his discoveries was elected a perpetual member, and obtained the gold medal. We notice this circumstance as forming a striking contrast to the reception bis inventions and discoveries appear to have met with from another eminent Society. We alludę to his first experiments, determining in a manner equally new and important the laws of magnetic action, and leading to the simple prac, tical invention which has conferred on him 'such well-merited distinction. : The account of these improvements was altogether too trifling, too meagré, too theoretical, to deserve a place in the Philosophical Transactions. We shall also in the course of this article bave occasion to express our highest admiration at a particular section of the volume before uş. We are convinced that our tribute of praise is no other than what must be the general sentiment: yet we learn, in a note appended to the section in question, that the substance of it was placed in the hands of a distinguished philosophical leader in March 1822, and read before an illustrious Society on the 23d of May following ; but that at the time of publication (Oct. 1822) the Committee had not decided respecting its appearance in print, nor has it yet appeared, though two opportunities have now passed.
After the conclusion of that part of the work relating to the laws of magnetic action, and the subject of the local attraction of ships, our author has inserted the substance of a memoir on the effects produced in the rates of chronometers by the proximity of masses of iron, which appeared in the Philosophical Transactions for 1821, and of which we gave our readers some account in our review of the volume containing it. Nearly the same remark will apply to the next section, in which Mr. Barlow presents us with an account of his experiments respecting the relative magnetic powers of different sorts of iron and steel, and on the anomalous attraction of heated iron. In this instance, however, the accounts of the investigations, particularly on the former point, are given in much greater detail than they were, as inserted in the Transactions. Upon these portions of the work therefore it will be needless to dwell any further, and we hasten to give some account of the succeeding parts, where the researches described are of a nature wholly new. And if they be less important in relation to practical purposes, and im mediate subserviency to the arts of life, than those which precede them, our scientific readers, at least, will agree that ample compensation is made in the simplicity and generality of the theories adopted, in the beauty and universality of the analytic formulæ deduced, and in the profound skilfulness of application and fertility of invention displayed in the mathematical reasoning.
The second part of the work is devoted to the explanation of a theory of magnetism, from which formulæ are deduced, applicable to all cases of induced and terrestrial magnetism. The hypothesis which Mr. Barlow here proposes is founded on an extremely simple supposition—a supposition strictly conformable to all the effects and properties which magnetie bodies display, and grounded upon the most natural, indeed we might almost say the only idea we can form, in any thing like a tangible shape, of the nature of the magnetic principle. It is not however our intention to dwell needlessly upon this part of the subject: we will briefly state Mr. Barlow's notion of the natore of this mysterious agent, and then proceed to point out the deductions he makes from it. He considers
Magnetic phenomena as due to the existence of two Auids in a greater or less degree of combination, and such that the particles
of the same fluid repel and those of an opposite nature attract each other.
“ These fuids in iron bodies exist naturally in a state of combination and equilibrium, till that state is disturbed by some exciting
“ But if a body, already magnetic, i. e. one in which these Auids are held in a state of separation be brought within the vicinity of a mass of iron, such as is supposed above, the concentrated action of each fluid in the magnetized body will act upon the latent fluids in the quiescent body, by repelling those of the same, and attracting those of the contrary kind, and thus impress upon the latter a temporary state of magnetic action, which will remain only while the two bodies maintain their respective situations.
“ The quantity of action thus impressed upon the iron body will depend, ist. upon the intensity of the exciting magnet; 2dly. upon the capacity of the quiescent body for magnetism, or the quantity of those fluids contained in it; and, 3dly. upon the cohesive power of the iron; which latter quality determines the depth to which the exciting magnet is able to disengage the two fluids."
This supposition Mr. Barlow conceives will comprehend every case of the developement of the latent magnetism in any iron body by means of a magnet, whether natural or artificial. And he proceeds to the more immediate consideration of the case of terrestrial magnetism acting on spheres of iron; a case which presents several conditions tending to simplify the hypothesis, and to render the phenomena more susceptible of correct mathematical investigation.
In a sphere of iron then we may conceive, (agreeably to the fact of magnetic action being confined to the outer shell,) that every particle in that shell is acted upon by equal powers in the same direction, viz. that in which the terrestrial
magnetism acts. In this theory then the action of the whole mass, instead of being accumulated in opposite poles at the extre. mities of the mass, is composed of the resulting actions of a displacement in each individual particle of the shell. And the resulting centres of opposite action are indefinitely near to each other in the common centre of attraction of the surface of the body-a centre which, as the author remarks, is coipcident with the centre of action of the mass, in the case of spheres, but not in any other bodies.
He deduces from this theory a very clear and simple view of the forces in operation, and of the compound action upon the needle, resulting.-By an ingenious application of mathe. matical reasoning, he obtains very simple expressions for the forces in question, and thence deduces the measure of the deviation in the supposed general case of a needle freely suspended, so as to be at liberty to take any direction whether horizontal, vertical, or oblique. From this he proceeds to deduce particular formulæ for the deviation of a needle limited in its motion to the horizontal plane, and of one moving only in the plane of the meridian, in other words, a dipping needle.
In the former case he shews by a comparison of results that the deductions from his theory agree in the closest manner with the experimental determinations given by Mr. Christie ; whose researches are already known to our readers. He also shews that the same beautiful and simple laws of magnetic action which he in his former work deduced from experiment, follow directly from the formulæ here investigated.
The formula for the dipping needle was compared with experiments instituted on purpose. In this case also as close an agreement was found, as could reasonably be expected.
In these investigations the object was only to compute the deviation caused in a horizontal or dipping needle by the magnetic action of an iron sphere:- in other words to compute the angle which the resultant of the two forces under consideration makes with the natural direction of the magnetic force: in the 2d section of this part, the author extends his enquiries to finding the actual value of that resultant, on the intensity of the action of the sphere on the needle. This is easily deduced from the formulæ : and in order to compare the value thus assigned in different cases, with experiment, the vibrations of a magnetized needle are to be considered under the same point of view as those of a simple pendulum. Upon this principle Mr. Barlow compares the computed and observed times in which a dipping needle makes a certain number of vibrations, when in various positions with respect to an iron shell. Tbis method be considers one of the best tests for examining the correctness of the theory: the approximation of the results to equality is very close ; and this approximation is produced from the hypothesis of referring the whole effect to a compound central action, instead of temporary poles at the extremities of the mass, in the direction of the dip, as is supposed in the hypothesis of Coulomb.
“ This point,” says Mr. Barlow, “I am the more anxious to establish, in consequence of its immediate connexion with the method I have proposed for correcting the errors of a ship's compass, which has been objected to on the ground that according to the theory we have been controverting, the central action of all the iron on board would not remain constant under all dips and in all parts of the world: but if the hypothesis I have advanced be correct, then