1629.] Col. Macdonald on the Two Magnetic Poles. Sir Lewis Dyve was one of the members for Bridport in Dorsetshire. On the assembling of the Long Parliament in 1640, he was again returned*; but was afterwards "disabled," probably in 1643, when several members were under that term expelled, for their then holding commands in the King's army. Although Sir Lewis does not appear as a speaker in Parliament himself, yet we find he was active in 1641 in publishing the speech which his halfbrother Lord Digby had made against the condemnation of the Earl of Strafford, and which was afterwards publicly burnt by the common hangman. It was delivered on the 21st of April that year, and on the 15th of July the House resolved, "that Sir Lewis Dyve and John Moor, as also Thomas Parslow, printer of the said speech, are delinquents, in printing and publishing thereof." Lord Digby, in his "Apology," issued in Jan. 1641-2, states, that he "did not only find that it was unfaithfully reported and uncharitably represented, but was informed that copies went abroad of it, so falsely and maliciously collected, as made the whole speech a justification of my Lord of Strafford's innocence; and Sir Lewis Dyve, having heard of such a copy in the house of a citizen of good quality, where he heard me mentioned as a person fit to have his name fixed upon posts, that it might be torn to pieces by the people, upon that reason earnestly desired me to give him a true copy of what I had said in that argument, which I did; and he forthwith gave directions for the printing of it, without any privity of mine." (To be continued.) MR. URBAN, J. G. N. Summerlands, Exeter, July 1, 1829. THE centenary of your valuable Twork has nearly arrived; and be ing peculiarly distinguished by ancient lore, without excluding useful scientific disquisition, a complete set, when obtained, constitutes a desirable article in every select library. Few subjects are of more real national importance List of that Parliament in Cobbett's Parliamentary History; the list of Burgesses in Hutchins's Dorsetshire, in which the two former elections of Sir Lewis are found, gives other names under 15 Car. I. 93 than the science of magnetic variation, on which a series of papers appeared in the Gentleman's Magazine. Permit me to add one, including some farther progress, sanctioned by observations and experiments. The advancement of a science still in its infancy, and on the establishment of which navigation and commerce mainly depend, is much retarded by the erroneous supposition of the existence of a multiplicity of magnetic poles. As it has been recently stated with confidence that the alleged position of a north-east pole has been confirmed by the observations of an eminent philosopher in a remote quarter of Eastern Russia, it is the object of this paper to disprove the sup position, and to attempt to make out that there can be but two magnetic poles, viz. the north-west and southeast; that is to say, one in each hemisphere. It is necessary to give an equatorial projection of the sphere, in order to elucidate clearly the object in view, and the demonstration will, cæteris paribus, be equally applicable to the southern hemisphere. It may be previously requisite to observe, that on several parts of the earth, in whatever direction they are approached, the needle is attracted by magnetic strata. In the northern hemisphere, these have a south polarity above, and a north below, with a reverse effect in the other hemisphere. The iron and guns of ships act similarly, in occasioning the local attraction of the needle, on the due knowledge and application of which life and property greatly depend; and this may induce me hereafter to give you my studied view of so very important a branch of magnetism. As one instance of local terrestrial attraction, the variation at the Falkland Islands has altered only a degree and a half since it had been first observed there; and this small attraction is to be ascribed to the constant movement of the north-west pole eastward, and of the south-east westward, at the rate of half a degree annually, and amounting only to a few miles, reckoned in the high latitude of the position of the eccentric curves in which they manifestly move. Eminent philosophers have situated each his pole in the south-west quarter, in order, by their supposed action, to account for the nearly stationary variation; but celebrated navigators, who have approached 24 Col. Macdonald on the Two Magnetic Poles. the supposed sites of these poles, have not found any real indication of their existence. In the following Figure, projected round the earth's north pole to the parallel of 70o, are situated the gratui [July, tous poles assigned in the northern hemisphere, by great philosophers, in support of their relative theories; and I shall endeavour to shew, that they are devoid of magnetic effect, and consequently of existence. Over N, the north pole of the earth, is drawn LNS, the meridian of London, being the line of no variation in 1657, because the needle at L, London, pointed due north to the magnetic pole S, corresponding to its real place within the earth. At P, on the west side of the figure, is situated the north-west magnetic pole, by approximations, in the year 1820, at the intersection of 70° north latitude and 100° west longitude. The pole is moving eastward in a curve probably more eccentric than P««« P. In order to discover the nature and position of this curve, it is now well understood, that at intervals of a few years, the site of the pole must be found at the positions, by means of the dipping-needle; and thus also its real rate of movement will be ascertained. This now can be effected easily, by the short run from the mouth of the Mackenzie, or Coppermine River. After former repeated. 1829.] Col. Macdonald on the Two Magnetic Poles. attempts, the enterprising Captain Ross has but a slender chance of getting through Regent's Channel, contiguous to the magnetic pole. This spirited navigator will try to get through one of the large channels on the north side of Barrow's Straits, and if he finds an open sea, he may plant the British flag on the pole of the earth; or he may steam along the north side of the Georgian Islands, and find a passage down to Behring's Straits*. No time ought to be lost in finding the points, because the pole is moving on to the inaccessible regions. It is of the most essential moment to navigation and commerce, that the place of the pole should at all times be known, in order to calculate the variation, and to find the local attraction of a ship, which is the difference between the observed and calculated variation, allowing for a small attraction by the more remote magnetic pole. PwN is the meridian of the northwest line of no variation, at the above period; and Ny P is the north-east line, running through western Russia and along the Coromandel coast. On this meridian in the southern hemisphere, a little to the south of the equator, a west variation arises, because the south extremity of the needle there is attracted eastward by the south-east pole, causing the north end to incline westward. For the same reason, in moving southward from the 25 The equator, the west variation increases, and would attain its maximum at the south pole of the earth. Moving thence, on the same meridian, the variation would become nothing, in approaching on the west side of the earth to the north-west_pole. celebrated magnetician, Mr. Churchman, supposed that the pole moved under a parallel of latitude, as PmKP. Were this the case, the present west variation would increase manifestly, till the pole arrived at the tangetical point m, when the angle NLm would express the angle of west variation. After this, it would decrease to nothing on the meridian of London, as in 1657. But as the variation was at its maximum in 1817, or 24° 17', and has been since decreasing, it is evident that it cannot be moving with its pole, under the parallel PB m. The pole cannot be moving in a straight line, or in a curve, under the meridian of no variation PNP, because all living under such meridian would have no variation at any time, a thing not a fact. The pole cannot be moving in a straight line on the north or south side of the pole of the earth, as, for instance, in the line Pro; because those living at P and o would always have the same variation, rPN or roN; excepting when the variation would vanish, when the pole was directly under their situation, P and o. On every other situation on this line, such Captain Ross, having determined once more to attempt the discovery of the NorthWest Passage, lately equipped a steam-vessel, called the Victory, entirely at his own expence. The steam power employed is on a wholly new principle; being so contrived as to combine every advantage of steam-power, with perfect capability as a sailing vessel. The boilers in use occupy so small a space, that they are fixed between the engines; the consumption of fuel is one-half, and the weight of the engines only one fourth, of those generally in use. Another advantage is, that a chimney may be dispensed with, which leaves the deck, masts, and rigging wholly unencumbered. The last intelligence from Captain Ross was dated Loch Ryan, June 14th and 15th. It appears that the engineer of the steamer was so severely hurt by the rod of the piston, that Captain Ross was obliged to amputate his arm. On joining the John (a vessel that was to have accompanied him) Captain Ross found that great discontent prevailed among the crew of the vessel; and upon his going on board, the crew individually refused to heave the anchor. Captain Ross then goes on to state, On my saying that my men (i. e. the crew of the Victory) should do it for them, the cowardly vagabonds took to the boats, and landed amidst the hisses and execrations, not only of the gallant crew (who gave three cheers, and said Let us go without them'), but of the villagers, who assembled to witness the landing. A scuffle took place between the captain and the crew; and in their attempting to take one of our boats to escape with, one man was knocked overboard; but no lives were lost. I immediately decided on removing what stores were necessary to complete us from the Johu, and to proceed without her; which has, I am happy to say, given my brave fellows universal satisfaction. They answered my determination with three cheers, and declared they would follow and stick to me wherever I went. We are all in the highest spirits; the wind is fair, the engine, bellows, and boiler, are all in repair, and the anchor is now up. Farewell!" GENT. MAG. July, 1829. 26 Col. Macdonald on the Two Magnetic Poles. as r, the variation would be east or west, according as the moving pole happened to be cast or west from the position r, where there would be no variation, when the pole would be directly under such site. If the line Po be supposed to be continued eastward and westward, to where the south-east pole begins to set sensibly, the variation on such continued line would be always the same, which is a case not known to exist. It must, from these statements, be concluded, that the magnetic poles move in some eccentric curve, such as P≈≈ P. Churchman, who made the period of a revolution of the pole 1096 years, instead of what it appears to be, vis. 720, situated his north-west pole in 1340 west longitude, and 580 north latitude, or nearly at c of the figure. This situation on the west coast of North America, has been frequently visited by navigators, without any indi cations of such pole by the dipping needle; and were it real, the horizontal needle would have pointed south, when passing through the Sound close to the south of its imagined site. The eminent mathematician, Eoler, placed his north-west pole at e of the figure, in 76° north latitude, and 96° west from Teneriffe, or nearly in 113° west longitude. This position, about seven and a half degrees from the real magnetic pole, and north-west from it, was ingeniously imagined, and enabled that great man to account tolerably for the variation in Europe and America. Captain Parry's first voyage completely disposes of this pole, as Winter-Harbour in 74° 47′ 13", and 110° 49′ longitude, was within 1° 12' 47" of the latitude of Enler's pole, and but a little on the east side of its meridian. Now it is evident, that thus situated, the north end of the needle must have pointed to such pole; whereas there was a variation of 127° 47′ 50′′ east, shewing that the north extremity of the needle was attracted by the real north-west magnetic pole. Professor Hansteen situates his pole in 1769, 19° 43' from the north pole of the earth, and in longitude 259° 58′ east. He makes it to move east ten or twelve minutes annually. Its place is nearly at a in the figure. From 1769 to 1819, fifty years, at the rate of twelve minutes of annual movement, would situate this pole in 90° west longitude, and 79° 13′ of latitude, at the period [July, when Captain Parry crossed its meridian. Had such pole existed, the needle would have pointed to it on this meridian, supposing it to be the real magnetic pole. The needle did not point due south, till the discovery-ships arrived on the meridian of 100° west longitude; and had the Professor's pole existed, the needle, influenced by both poles, would have given a variation of probably 145° east, instead of 180° on the real meridian, or line of no variation. Monsieur Biot, a philosopher of considerable eminence, has located a magnetic pole in 70° north latitude and 720 west longitude; and the great Dr. Halley, second only to Newton, placed a pole nearly in the same situation in Baffin's Bay. It is only necessary to say, that the alleged sites of these supposed poles have been explored, without finding any trace of their reality. At the south east cape of Greenland, in latitude 70° and longitude 23" west, and nearly at K of the figure, Professor Krufft situates his north-west pole. In frequently crossing the meridian of this imaginary pole, and so near that the horizontal and dipping needle must have been acted on, no such effect appeared. If a line be drawn from London to the place of four of these poles, it will pass much to the north and east side of the place of the real north-west_pole P; and a line from London to Biot and Hansteen's pole, will pass a little to the southward of it. On magnetic principles, therefore, the needle at London ought to point to the eastward of the real pole. But how stands the fact? It is, that by means of the colatitude of London, the co-latitude of the pole, and the difference of longi. tude, the calculated is 1° 13' less than the observed variation; or, in other words, the needle points to 1° 13′ to the southward of the real magnetic meridian; because the south-east magnetic pole, acting on the south end of the needle inversely as the square of the distance, attracts it eastward, and increases the calculated to the observed variation. The distinguished authors mentioned did not, in point of fact (with the exception of Halley and Hansteen) suppose the existence of more poles than one in each hemisphere; and they placed their pole or poles so as to correspond with their respective theories. On the east side of the meridian of 1829.] Col. Macdonald on the Two Magnetic Poles. London Dr. Halley situated his second pole, in 76° 30′ north latitude, and 30° east longitude. Captain Parry passed within a few meridional degrees of this position; and if such pole had been there, the north end of the needle must have been attracted by it into a considerable east variation; instead of which, it continued west. Other navigators passed near to the imagined site of this north-east pole, without perceiving any symptoms of its actuality. Hailey's imaginary pole is placed in the figure at H. We have one other pole to dispose of, which is the learned Professor Hansteen's, said to be in a situation where it is utterly impossible to approach it, viz. in 85° 49′ north latitude, and 101° 30′ east longitude; being only 4° 11' from the north pole of the earth, and nearly at h in the figure. In Captain Parry's last publication, a small decrease of the west variation, experienced in proceeding to Spitzbergen, is ascribed to the inAluence of this supposed magnetic pole in the north-east quarter. Supposing a ship to move eastward on any parallel of latitude, as that of 80° for instance, and to arrive at 1 of the figure; here the west variation would be the angle NIP, formed by the magnetic meridian IP with the meridian IN, of the place of observation. Let the ship move on to 2, where the variation N2P will be manifestly less than that at 1. At 3, the variation will be still less; and at P of the meridian NyP, or eastern line of no variation, it will be nothing; because the north pole of the earth, Ñ, and the real north-west pole, P, on the west side of the figure, are under one and the same meridian, PwNyP. After passing this meridian, an east variation would arise; and which, at 4, would amount to the angle P4N. At 5, the variation would be greater; and at o, it would be indicated by the angle PoN. Were the Professor's the only pole in the northern hemisphere, the variation at o would be only the small angle Noh. The scientific world are greatly indebted to Professor Hansteen for having lately made a laborious journey into Siberia, in order to elucidate important points in the useful science of magnetic variation. He is said to have made satisfactory observations at Irkutsk, in latitude 52°, and longitude 104° east. The north-west magnetic pole is considerably to the east of the meridian of 27 this place, and the east variation arising from its position, is much diminished by the action of the south-east pole, still nearer to the meridian of the above town in East Siberia. This would render the variation very little, but still it could not be ascribed to the imagined pole lying on the west side of the meridian of Irkutsk. All over the oriental quarter, and considerably beyond the equator, the variation is little, on account of the manifest counterac tion of the two poles, on the same side of the meridian of places under the above magnetic circumstances. Supposing the existence of a pole at h, it is evident, on known magnetic principles, that the needle at London could not point to the pole at P. It may be sufficient here to mention the result of experiments made with two magnets of equal power applied to the place of the real and of the supposed pole, in reference to a sensitive needle resting on the place of London. The variation given varied a few degrees, as might be expected; but the medium gave the angle LNn as the variation: but as the needle at London points in an angle somewhat greater than the calculated variation, NLP, it amounts to positive demonstration that the north-west is the only magnetic pole in the northern hemisphere; and similar reasoning applied to a similar figure for the southern hemisphere, would, mutatis mutandis, yield an equally conclusive result. On the site of the magnetic pole the dipping needle will stand perpendicular, in continuation of the semi-diameter of the earth, and on some part of which, within the earth, the real pole, or magnetic power, is situated. After much consideration, it occurred to me that the depth of the pole within the hollow sphere of the earth, might be calculated trigonometrically. The difference of latitude between the place of the pole on the surface, and the place where a dip is taken, gave the angle at the centre. The complement of the angle of the dip to 90° gave the acute angle at the circumference. Having thus the three angles of a triangle within the earth, and the radius as one of the sides, the part of the other radius, intercepted between the true place of the pole and the centre of the earth, was calculated. By subtracting this from the semi-diameter on which the pole is situated, |