cordances in the variation on ship lished. In this valuable tract* the board-a change in the direction of author, in the last section, has, in a the ship's head. Having ascertained variety of examples wholly incompathis most important truth, it soon oc- tible with the supposition of truth, curred to him, that a local attraction completely exposed the fallacy of this must exist in the ship; which, in con- rule-which Captain Sabine fully cornexion with terrestrial magnetic at- roborates by observations made during traction, acted on the magnetic needle, the late arctic expedition ; and which, when placed at the binnacle, with a indeed, is the only thing of the least compound force; and, therefore, he consequence done in that voyage that found by experiments, that in the has added to our previous knowledge porthern hemisphere, when the head on this subject. As to the unintelliwas at west, this combined attractive gible paper written by Mr Scorsby, and power drew the north end of the inserted in the Philosophical Transacneedle to the west, and in the south- tions for 1820, Part I., we are comern hemisphere, to the east, of the pelled to say, it is wholly undeserving true magnetic meridian ; This differ. of the least attention, and serves no ence, produced by local attraction, he other end whatever than to bewilder denominated the Deviation.' the reader into a labyrinth of useless Finding the maximum deviation in experiments, which, we much ques both hemispheres, when the ship's tion, whether the author himself head was at west or east; and that rightly understands. How very difthe needle stood right when the head ferent are the experiments and deducwas in a direction with the magnetic tions we now intend briefly to analyse ! meridian-north and south ;-what Mr Barlow, author of the work bewas the proportion of deviation, he fore us, an able mathematician, and asked himself, at the intermediate one of the Professors of the Royal Mipoints, between the east and west and litary Academy, Woolwich, sensible magnetic meridian? After much la- of how very much real importance a bour and consideration, it appeared to formula, founded on correct principles, him, “ that the errors produced by for correcting the deviation produced local attraction should be proportion- by a change in the direction of the ate to the sines of the angles between ship’s head, in all approachable latithe ship's head and the magnetic me- tudes, would be to science and naviridian ; and, therefore, in order to gation, and, indeed, to mankind in find this proportion, it seemed pro- general, has at length arrived at the bable the following Rule would be conclusion, after a long, laborious, found applicable in all parts of the and patient investigation of the laws world, viz. “ that the error produced of magnetic attraction, which his siat any direction of the ship's head, tuation and place afforded the most would be to the error at east or west, ample opportunity and means for exat the same dip, as the sine of the angle periments, no less honourable to himbetween the ship's head and magnetic self than beneficial to science and pracmeridian was to the sine of eight points, tical navigation. Before we introduce or radius. our readers into our author's workCaptain Flinders dying soon after shop, it may be proper they should his return to England, we see no fur- clearly understand what Mr Barlow's ther attempts made either to verify or ideas were, on this subject, before he overthrow the accuracy of this rule, commenced operations; and the theory until 1807, when a small practical on which all his future hopes dependwork, entitled," an Essay on the ed. We shall transcribe his own Variation of the Compass, by W. Bain, words :-“Since the iron of the vesmaster in the royal navy,” was pub- sel," says our author, " and the com The importance of this work, in a practical point of view, having excited considerable interest, we refer such of our readers as may not have had an opportunity of seeing she book itself, to Brand's Journal of Seience and the Arts, No 7,-Monthly Review, November 1817,-and British Review, November 1819,—who have each treated the work and the author in a popular manner. Indeed, it has always been to us a matter of regret, that Mr Bain should not have been employed in the late Arctic expedition. + That this paper should have been inserted, and that of Mr Barlow's rejected, in that No of the Philosophical Transanctions, surprises us a geod deal, and cannot well be accounted for. tan. I.ech reosa pass must be supposed to maintain the compass ; “I elevated the ball," he same relative position, with respect to continues, " till its action was impereach other, during the voyage, I ima- ceptible"; and then gradually lowergine it to be possible to place a single ing it, I noticed the deviation at various ball of iron, equal to the whole mass, altitudes of the ball, with the compass in a certain situation of the ship, when of each point of division on the cirits effects upon the needle would be cles ; observing also very accurately the same as that of the iron in its dis- the height or depth of the centre of tributed state; or what amounts to the ball above or below the pivot of the same thing, that all the forces acte the needle, when the deviation was ing on the needle in the actual state zro. These results, indeed, were the of the iron, may be reduced to a sin- only ones applicable to my present gle resultant. Í then assume, that a inquiry; and from them I ascertained less mass of iron (having its entire at that the several parts of no action traction, or resultant, in the same were all situated in one plane; the line as the former,) may be approxi- inclination of the plane itself to the mated so near to the compass as to horizon being found nearly equal to produce an effect equal to that of the 20°, declining directly from the magiron of the vessel, whereby the tan- netic north point to the south. This gent of the angle of deviation may be plan is, therefore, either exactly or very at any time doubled, and hence the nearly perpendicular to the direction of deviation itself determined. Under the dipping needle. The formula used this point of view, however, a slight by our author for computing the recomputation would be requisite ; but sult, which will be found in page since the tangent of small ares have 19, and which approximates nearly to very near the same ratio of the ares that found by experiment, is this :themselves, we may suppose the angle itself doubled by the experiment, and sec, a. When I hence the deviation ascertained by ob- denotes the inclination, r the radius servation only.” Preface, p. 5 and 6. of the circle, which, in the present Though our author's first experi- instance, was twenty inches, h the ments rather involved him into diffi- observed height or depth of the cenculties, at least proved nothing at the tre of the ball, and a the angle from time, yet it is necessary the reader the east or west points of the circle. should know the apparatus he worked The mean result of all those calculawith, as well as the method by which tions, which involves too many figures he worked with it;-" I began,” he to be inserted in this outline, gives says, p. 3, “ by describing, on a plat- 19° 24'. form, several concentric circles, from The different manifestations india eight to sixteen inches radius, draw- cated in these important experiments, ing through the centre a line, in the between the needle and the attractive direction of the magnetic meridian ; I power of the ball, at different angles, then set off my east and west points ; horizontally and perpendicularly, inand, lastly, divided the whole circle duced our author to believe, that there into equal parts of 10° each." With were in every ball of iron two planes, the compass over the centre of these in which the compass may be anyconcentric circles, he passed round where posited, without being influsuccessively, on each circle, several enced in its direction; the one that of shells of different diameter; which, no attraction, and the other the veras our author anticipated, produced tical plane, corresponding to the mage results at the time wholly inexplicable. netic meridian. In consequence of At length, however, our author, have which, he conceived an ideal sphere ingexchanged his platform for a strong to be circumscribed about the ball of table, divided according to the points iron ; and assuming the circle of no of the compass, with a circular hole attraction as an equator, and the poles of ten inches diameter in the centre, of that circle as the poles of the sphere, through which a ten inch shell was, he imagined circles of latitude and lonat pleasure, made to pass by means of gitude to be described round the ball a block and pully, he again commen- in several circles, keeping it always at ced operations, by passing the com- the same distance from the centre ; pass on the circle round the ball, in- and, therefore, when these ideal circles stead, as before, the ball round the of latitucle and longitude were core rectly ascertained by calculation, and to the Board of Admiralty, was acverified by experiment, as particularly knowledged ; and the pleasing and described page 21, and to which we friendly way Sir George Cockburn, must refer our readers, he succeeded one of the Admiralty Lords, and Mr in establishing the law of deviation as Croker, the secretary, offered our auit respects the latitude ; namely, that thor all their interest in the further the tangents of the deviations are pro- prosecution of his experiments. portional to the rectangle of the sine It is many years since we knew and cosine of the latitude ; or to the Captain, now Admiral Cockburn, sine of the double latitude, which is the and our readers will easily conceive, same thing. In the same way our au- that the man who, with the same thor establishes a like law of elevation, activity and thirst after professional as it respects the longitude; and, to knowledge, could ascend the mastexplain himself more fully, he has head, and there detect stupidity in given two diagrams, one describing the the adjustment of a sky-sail studdinglaws of deviation as it regards the la- sail, as descend the hold, and super. titude, and the other as it regards the intend the stowage of a water-cask, longitude. must have felt exquisite pleasure and With regard to the laws of attrac- delight on entering our author's worktion as it regards the mass of iron, our shop, and there behold, by accurate author has, by a great variety of ex• experiment, the true cause producing periments, verified by computation, the extraordinary anomalies in the clearly demonstrated, that the power variation of the compass. Mr Croker, of attraction resides wholly on the sur- too, a man of science, liberality, and face, and is independent of the mass; erudition, could not help feeling, on or, in other words, that the tangents this occasion, much gratification, and of the deviations are proportional to the it is the highest encomium we can pay cubes of the diameter, or us the į power to the professional knowledge of the of the surface, whatever may be the one, and learning of the other, to have weight or thickness.--Page 48. it in our power thus to record an in The striking confirmation of the ex- stance of such noble generosity and istence of the plane of no attraction in kindness in behalf of aspiring talent the most irregular masses of iron ; and and genius. that the power of attraction resides But, to return from this digression, wholly on the surface, and is inde- our author procured, for his next expependent of the mass; as exhibited by riment, an iron 24 pounder, mounted experiment, and confirmed by com- on a platform which admitted of its putation, must have gratified and en- being traversed through an entire cir. couraged our author in his toilsome cumference; the tracks at the bottom and unbeaten path ; and, therefore, to running over a circle ten feet six inches put the matter beyond all dispute, he diameter, divided into 32 equal parts determined to verify his former expe- corresponding with the points of the riments by others of a different na- compass. A piece of wood, projecting ture, and on a much larger scale. Ac- four feet from the muzzle of the gun, cordingly he addressed a letter to Sir for the compass to stand on, was made William Congreve, requesting permis- to fit exactly the bore of the gun, on sion to pursue his inquiries in the re, which the compass could be moved to pository at Woolwich; and, it gives us any distance, at the time of experigreat pleasure to say, that the very polite ment. As it is impossible to make and handsome manner in which this room for the results, we shall just now communication was answered, reflects say, that they fully corroborated the acthe highest honour on the character of curacy of the results given by former that worthy and ingenious gentleman. experiments; and that, in the present How contrary was the behaviour and instance, the difference between the conduct of the Royal Society to our observed and the computed results is author on a similar occasion, will best so very trifling, as to be almost imbe seen in Mr Barlow's own language, perceptible. It is necessary, however, page 12; which will appear totally in the reader should know the formula comprehensible when contrasted with by which our author obtained results the answer of Sir William, and the bearing so close affinity with those manner in which a memorial, address- given by experiment. - The comed by our author, for the like purpose, puted deviations,” he observes, “was sin. 22 cos. The A. obtained by first finding from the ob- 7° 28', cos. 10° 48': tan. 6° 30': tan. served deviation the mean ratio or 2° 30', which latter is exactly the devalue of A= sin. 2 a cos. then using viation found by observation in Table tang. A. 11.” to which we must again refer our it as a constant co-efficient (A) in the readers. expression tan. A=" Our author has applied the above same may be likewise done by saying. rule in the computations of deviations “ As the rectangle of sin. 2 a cos. v, found by Captain Sabine on board (corresponding to any position of the the Isabella off Shetland, where the compass), to sin. 2 a cos. „, (answering dip was 74° 21', with great success ; to any other position), so is the tan- since the greatest error between Cape gent of the deviation in the first in- tain Sabine's observed deviation, and stance to that of the sound." the computations of our author, only “ For example—the latitude and amounts to 49'. longitude corresponding to 45°, or NE, From the singular discovery, “ that is latitude 13° 30', longitude 43° 18'; the power of attraction resides wholly and the same answering to one point in the surface of iron bodies, and is from the east, is 3° 44', and longitude independent of the mass,” our author, 10° 48' : therefore, by one of those happy ideas so pecu“ As sin. 27°, cos. 43° 18': sin. liar to genius, conceived the possibili EXPERIMENTS 93. ty of ascertaining the correct deviation above table, our author assumes, and in all ships, in all positions, and in all with some degree of probability, that places, by simple observation only, and when the whole quantity of deviation independent of computation. With is once ascertained by swinging the this view, he ordered a frame-work to vessel , the navigator, by attaching a be affixed to the gun, which should plate of iron to the binnacle in a plane project beyond the compass, whereby with the centre of greatest deviation, he could suspend a ten inch shell in or local attraction, which, he says, will any required position with respect to be found in most vessels at an angle the centre of the needle. Having fix- between 20° and 60°, but which in the ed the ball in the required situation, Isabella he found by computation to he repeated his first course of experi- amount to 65°, which he considers an ments, which we have already noticed, extreme case-may at all times, and with the ball attached, by traversing in all places, find, without computhe gun through the entire circle ; tation, the exact deviation, by turnand as the results of this experimenting round the binnacle, with the plate are of the utmost importance, in a of iron attached. In like manner our practical point of view, consequently to author made his experimental results navigators, we shall give them entire. inserted in the above table: Or, in his From the results indicated in the own words, “ supposirg now this first approximate angle to have been found, pect, that our author will find the then the plate must be fixed, so that true cause of the apathy and indifferits centre of attraction inclines from ence shewn him and his experiments, the pivot of the compass, at about the to which he alludes, p. 12, and to same angle; then, turning round the which we have also shortly adverted, binnacle, point by point, observe the to lie within this hitherto mysterious deviation caused by the plate ; and if circle. these correspond with those given by It would be extremely unjust, howthe vessel, the plate is rightly adjust- ever, for us to assert, or even to insied; but if not,* (as is most likely to nuate, that this censure attaches happen,) such triding changes may be wholly to the venerable and ilmade in its position, which will be at- lustrious president. His liberality tained in a few trials.”—P. 87. Our and candour are recorded over the author has given a particular descrip- earth; and we are ready to acknowtion of this plate, and the most proper ledge, that no man, at any one period of method of attaching it permanently to our history, has done more for the adthe binnacle; but we had proceeded vancement of science and protection this far, when our limits admonished of genius than Sir Joseph Banks. us of the necessity of terminating our But his infirmities, which, from the own remarks; we must, therefore, re- natural course of evnts, we are sorry fer the reader to the work itself on to say, must be many, probably leaves this interesting head of the book. For room to suppose, without invidious the same reason we must reluctantly interpretation, that his confidence is decline entering, as we fully intended, often abused; for experience demonupon our author's beautiful theory, by strates, that in every human instituwhich he determines the laws produca tion there are always swarms of uning the diurnal variation of the com- profitable drones, proportionate to the pass; which, from the analogy of ex- magnitude and importance of that inperimental and computed results, ap- stitution, thrust into place by influence pears unquestionable ; and which, be- and power, who are ever found to fating bottomed on correct philosophical ten on the credulity, or merits, or principles, seems to us the only theory genius, of meek, lonely, and unsuse deserving of the least attention. pecting minds; and, since this is un Should the truth of this remark be avoidably the case, it would be foolish admitted, which indeed appears incon- to suppose, that an hemisphere, so trovertible, when clearly and dispas- brilliant and extensive as that which sionately understood, what, we should surrounds the chair of the Royal Solike to know, will the worshippers of ciety, can be altogether free of such M. Biot say to Mr Barlow's able, but harpies. modest, exposition of an error of 7° in Our love of justice, and respect for that theory, regarding the laws regu- the genius and merit displayed in Mr lating the dip, or inclination, of the Barlow's valuable book, have impelled needle, which the French mathema- us to say this much; for it would be tician has had the address to impose a strange dereliction of our duty, so long on the credulity of his yota- which has hitherto, we trust, been imries ; and which, by our author's theo- partially discharged, did we pass over, ry, a discovery Biot never dreamed of, without animadversion, this glaring bears a close affinity with the laws re- instance of ungenerous conduct to a gulating the daily variation and de- man, evidently of great abilities. What viation of the compass. After this ex- we have said, however, will probably posure—for though the Frenchman is produce little effect towards restoring an able mathematician, yet he is no that Society to its original excellence, experimenter-we could fain hope, or Mr Barlow to its future protection that the eyes and ears of the venerable and regard; be it so. But we can chairman, or president, of the Royal assure our author, that if he only conSociety, that monument of British tinues his studies with the same perglory, will now be open to English severing ardour every where manifestmerit and justice: and, in future, pre- ed throughout his book, which we vent the malignant influence of foreign earnestly recommend to the serious counsels swaying the important de- consideration of the public, his incisions of that illustrious society. We dustry and talents will very soon renhave more reasons than one for mak- der him altogether independent of ing this remark; and we strongly susa that or any other Society whatever. 7 |