« AnteriorContinuar »
Col. Macdonald on Magnetic Variation. [June, He who erst the city freed
of the needle moves towards the west, at From the winged virgin's power, Fort Marlborough and St. Helena, which Charge him not with such a deed,
are situated to the south of the equator, • Doom'd to Misery's joyless hour. the movement is directly opposite, namely,
to the east." _“ It is evident that the obMr. Urban, Summerlands, June 2. servations made to the north of the line, O science can be more intimately of the southern hemisphere present, like
agree with those of Europe ; and that those perity, than that on which Navigation opposite movement.” and Commerce indispensably depend ; and, therefore, such facilities as the I ascribed these effects on the needle extensive circulation of a work of re
to the action of solar heat on the magpute like yours must give to the form- netic poles, rendered weaker or stronger ation of a true Theory of Magnetic Va- at different times within the twentyriation, are manifestly subservient to four hours, according to the presence so essential an object. Further obser- or absence of the Sun: hul recent esvations and experience enable us to periments evince that the direct heat rectify and improve this great subject, of this glorious luminary is the more in the rapid progress it is niaking; and efficient cause. Professor Morichini with this view i furnished my esteem- has discovered that, when unmagneed friend and the intrepid navigator, tised needles were placed for a short Captain Parry, with my conceptions time in the violet rays of the spectrum of this infant science, in order that this formed by passing the Sun's rays intelligent character, and the able men through a prism, polarity was acquired. about him, might consider my posi- Consiliachi and Beard tried this expetions, and suggest what might be cal- riment withoat succeeding; while the culated to advance it to its much wish- ingenious Mrs. Somerville has rendered and ultimate stage.
ed her name famous by the manner in Some important discoveries recently which she succeeded, which was by made relative to the magnetic effects covering the half of her needles with of solar heat, elucidate clearly one cu- paper, and by placing them in the rious department of the science of Va- violet rays of the spectrum given by riation, 'first noticed by Graham in the prisın, the uncovered part became 1722, and by Canton in 1756, who a north pole, and more readily than marked the daily variation, without when the needles were situated in the being enabled to account for it on le- green, blue, or indigo rays. This phigitimate grounds. I first observed this losophical lady covered one balf of a description of variation in the south- needle with paper, and enveloped the ern hemisphere in 1794-5, at Ben- other in green ribbon. Thus prepared, coolen, and in 1996 at St. Helena ; and exposed to a strong Sun, it became ascertaining that it moved eastward at magnetic, and still more readily, when the very time when it moved westward the Sun's rays were concentrated. She in the other hemisphere. If I had found that a needle half covered with made these observations as far south paper, and placed under a blue glass, as London is north, I should have exposed for some time to a strong Sun, found the quantum of diurnal varia- acquired polarity. tion similar; and as I stated at the Mr. Baumgartner of Vienna polishtime, it would have been found greater ed zones on an oxidised or rusty needle; with a needle on the more delicate sus- and after having been placed for some pended principle. My account of this time in the Sun, the polished zones beinteresting phenomenon, printed in came north poles, and the oxidated or our Philosophical Transactions, is re- rusty parts south poles. On trying lonferred to as follows, in the proceedings gitudinal polished stripes, no effect was of the French Academy of Science.
To Mr. Christie of Cambridge, mag. “ Mr. Macdonald's labours led to two important results. The first (wbich every
netic science is much indebted. He philosopher appears to have adopted) is, has discovered that the uscillations of that the daily variations between the tro
a magnetic needle stop sooner in the pics have a sepsibly less extent than in Eu- Sun, than in the shade. He fonad rope. The second (to which less attention that the oscillations in the same time has been paid) is, that at the same hours at were fewer in the Sun, than under cowhich in our climate the northern extremity He ascertained, that the highes
1827.] Col. Macdonald on Magnetic Variation.
501 the temperature of the air, the more process. Supposing that the interventhe magnetic intensity diminished; tiou of ice might prevent arriving at the and by a series of well-conducted ex- actoal site of the South-east Pole, the periments, he makes it clearly appear oscillations of the needle where pro ihat the solar rays have a magnetic ef- gress might be interrupted, compared fect, independent of their heat. The with the same number of oscillations oscillations may be rendered slower or in the same time on the line of no vafewer in the Sun, by the expansion of riation in the other hemisphere, would the needle by direct heat; but from leave the distance between the place of the other experiments, it would ap- observation and the supposed 'known pear that the diurnal variation is occa- place of the come-at-able North-west sioned principally by the direct mag- Pole, equal to the distance between netic action of 'solar heat. At Port the place where the ice stopped pro.. Bowen, the greatest west variation was gress, and the real place of the Southwhen the Sun was west from the east Pole. For reasons stated, the place; and at Bencoolen and Si. He- needle would be shaded in both places lena, ihe maximum of east diurnal va- of observation, and the similarity of riation happened when the Sun was atinospheric temperature would be atsituated east froin these places. The tended to. needle returned slowly to the opposite It appears that the annual increase position, when the Sun ceased to be or decrease of the variation is unequal. on the attracting side.
arise from the effect of magThere can be no doubt now, but netic strata, and more apparently from shat the Sun is the great source of the situation of the two magnetic poles, magnetic power, communicated to the relative to the place of observation. magnetic poles through the two points from a medium of 396 observations on the earih, where the dipping needle taken by me at Bencoolen in 1794, on will stand perpendicular." The mag- a true meridian, the variation was netic fluid issues in all directions from 10 g east. In 1822, the Surveyor-gethese primary points; and thus the neral found there a variation of 1° 15' nearest extremiiy of the needle is at- east. Thus, in 28 years the annual intracted towards then in the inverse ra- crease has been there only fifteen setio
the square of the distance. conds. In 1794 the North-west Pole It is earnestly hoped that his Royal was about 37 degrees to the east of the Highness the Lord High Admiral will meridian of Bencoolen ; and the southdirect the highly important discovery east was 31 degrees to the east of it ; to be made, without which no theory and consequently, they neutralized can be established, viz. the real posis nearly the effect of each other, which tion of each magnetic pole. Suppos- will be the case for a long period to ing the approximated position near the come; and this accounts also for the truth, we have a spherical triangle, of very small and slow alteration observwhich two sides, the colatitude of the able in the variation all over the eastpole, the colatitude of London, and ern seas and islands during a long past the intercepted angle of 100°, being period. A most important science the difference of longitude, are given, must continue to float in uncertainty to find the angles at the base, one of and conjecture, till the rate of movewhich ought to be the variation there. ment, the nature of the path, and the This gives the calculated variation positive site of both magnetic Poles equal to 23° 17', less by jo 13' than shall have been ascertained. I men24° 30', the observed variation. Now tion both Poles, because some eminent the calculated ought to exceed the ob- philosophers, seeing anomalies of variaserved variation, because this last is tion in several places, have placed diminished by the action of the distant poles in various situations, in order to south-east pole on the south extremity ascribe to them such irregularities of of the needle at London. All this variation. This anomaly of variation proves that we have not as yet got the occurs in the East Indies, in Hudson's irue position of the North-west Pole. Bay, about Cape Horn, among the It lies not far from Coppermine River; South Zeiland Islands, and remarkaand by obtaining experimentally its bly about the Falkland Islands, where position, and the same ten years hence, Captain Basil Hall, in 1824, found an its path and real rate of movement will east variation of twenty-two degrees ; be clearly ascertained, and only by such while Commodore Byron, in 1764,
(June, made it 23° 30' in that very place; tion which takes place on the plomgiving only a degree and a half of dif met of a quadrant is not of a magnetic ference during 60 years. In 1824, the nature; but is referred to matter actNorth-west Pole was ten degrees westing on matter. The shell of the earth /from Port Egmont at these islands; is richly stored with metals for the use and supposing no South-east Pole to of man; and of these iron is the most exist, Captain Hall ought to have useful. The needle is acted on by found a small west variation. Again, metallic substances unequally situated. in 1824, the South-east Pole was nearly Where they abound, they acquire pounder the meridian of Port Egmont, larity; and philosophers imagine poles and being much nearer than the other, where the observed effect is due to consequently the west variation due to magnetic strata in many instances. the other Pole ought to appear reduced These supposed poles are placed in sior neutralized to nearly nothing. To tuations not sanctioned by experience, account for this anomaly of 22° east, or any convincing rationale. With where in Commodore "Byron's time the two KNOWN inagnetic poles, and also it ought to have been still farther with a knowledge of their effect on west, Dr. Halley and Professor Hans- metallic strata, the variation can be teen situate a magnetic pole in the rationally accounted for. It appears south-west quarter. Captain Cook, that more poles than two would dein 1774, went within a few degrees of stroy each other's effects, and would the assigned sites of these poles, and embarras calculation, and render it found no indications of their existence. uncertain, if not impossible. Supposing them, however, to be ac
As many have supposed that each lually as placed, and 10 attract the magnetic pole has its peculiar magsouth end of the needle so as to incline neric equaior, it is necessary to state the north end into the east variation at the real fact of the case, as a compoPort Egmoni, what must become of nent part of the theory as now underthe undoubted and known attraction stood. The true magnetic primary of the two poles actually operating? meridian is a circle on the globe, passThese poles acting as they do, con- ing through both magnetic poles; and jointly, would counteract ihe atırac- it crosses the terrestrial equator in 91° tion of the poles of Halley and Hans- 30' west, and 86° east longitude. The teen, and, as before, neutralize the east and west divisions of this circle twenty-two degrees of east variation will be unequal, because the magnetic to nearly nothing. But still this va- poles are not at present at equal disriation actually appears, and how are tances from the poles of the earth. we to account for it anong such dis. From the North-west Pole lay off the tracling attractions? It is well known half of the west division of this primary that the magnetic poles within the magnetic meridian, and it will extend earth impart magnetism 10 masses of to 16° south latitude, in 91° west loniron, iron ore, iron bars, &c.; and that gitude. From the north-west or southsuch magnetism in each hemisphere east pole lay off the half of the east diis of an opposite quality or name. At vision, and it will terminate in 16° 30' Port Egmont the south end of the north latitude, and nearly under the needle would point to the upper part meridian of 860 east longitude. The of a common fire-grate; and the north, first of these latitudes will give the to the bottom; while its action would farthest south, and the second the farbe neutralized at the middle or mag- thest north point of the magnetic netic equator of the mass of iron. equator, common to both magnetic Supposing these islands to be based or poles. The needle in every part of founded on metallic ore, or on mag- ihe primary inagnetic meridian will netic strata, it may be consistent with lie in tl:e plane of both magnetic poles, well-known facts to suppose that the and will have no dip in the above latimagnetic needle is attracted as above; tudes, being rendered horizontal hy and according as one end may be re- the equal attraction of the magnetic pelled, or the reverse, on established poles at equal distances. In every principles, the local attraction, unac- other part of the circle, it will dip tocountable by polar action, as stated, wards the nearest pole magnetic. In may be accounted for in the manner no other situation, excepting on this specified. Navigators call this the at- magnetic circle, will the needle point, traction of the land, but the descrip. at the same time, to both magnetic
1827.] On the Sazon Inscriplion in Leoninster Church. 503 poles. When the nearest magnetic as also the extreme points of northing pole is between any place and the and southing of the magnetic equator. nearest terrestrial pole, the meridian The position of these iwo places farpassing over that place and these poles, thest north and souh, will be found is called the line of no variation to such always by halving the west and east place; and this meridian continued unequal divisions of the primary magbeyond the terrestrial pole to the mag: netic meridian passing through the netic equator on the other side, will two magnetic poles round the globe. be a line of no variation to the inha- The ingenious galvanic experiments biiants living under it. On this line of Ærsted, Ampère, and Arago, evince the needle will not point exactly to the existence of east and west magthe nearest magnetic pole, because the pelic currents occasioned by the Sun other end of it is attracted by the more in bis course; and producing by the distant magnetic pole in the inverse action of magnetic caloric heat, the ratio of the square of the distance; and diurnal variation, always greater in therefore the observer' must move a summer than in winter. The main little to the east or west of the place magnetic current flows in every direc(according to the position of the inore tion from pole to pole, diminishing in distant magnetic pole), in order that intensity inversely as the square of the the needle, acted on by both poles, distance, and equal in strength on the may be in the true line of no variation magnetic equator, where the currents of such place. Supposing both mag- acting equally on both extremities of netic poles of equal strength or inten- the needle, renders it horizontal. sity, iwo conditions are requisite to This useful and interesting subject, render the needle horizontal, or devoid Mr. Urban, is attracting much attenof dip. The first is, that such place tion; and in order to elicit discussion must be at an equal distance from of a new science intimately connected both magnetic poles; and the second with public welfare, I state from time is, that a line drawn from each mag- to time what may tend to advance it netic pole to such place, should form gradually to an established theory, ou nearly an equal angle with its meri- which safety in the trackless deep" dian, or with the parallel of latitude, so much depends. as these angles are complements 10 Yours, &c. John MACDONALD. each other, to ninety degrees. There are only two places on the terrestrial
June 12. Y QUE
TOUR Saxon readers are much obtions. The one is at present in 80 east, liged to you and your Correspondand the other in 1720 west longitude, ent Ş. Y. E. for the republication of The magnetic crosses the terrestrial the Inscription found on the south equator at these points, where also of wall of Leominster Church. Nothing course the needle will be horizontal. can be more groundless than the oba If a line be drawn through these jections raised against the authenticity points, it will be nearly the present of this Inscription ; nor does it require situation of the magnetic equator, the support of a similar tablet of brass which is constantly altering, on ac. formerly affixed to a column of the count of the perpetual movement of Abbey Church at Glastonbury, as nothe magnetic poles. The magnetic ticed by Usher (Antiquitates, p. 9, ed. equator, as at present situated, passes a 1698). The age of the latter is uncerlittle to the north of Pharnambooco in tain; and it records an event which, Brazil; near to Cape Rocket, to the if true, has been so blended with the south of ihe entrance into the Red marvellous and the superstitious, as to Sea; to the south of Goa on the coast throw an air of suspicion over the of Malabar; 10 the south of Vizagapa- whole story. The facts recorded in lam; to the north of Cambodia; over the Leominster Inscription are not of the south of Manilla; through the Ca- this nature; and whether preserved in rolinas-Islands; over the equator, as brass or stone, or in any other way, is mentioned ; and over the north of the a point of no great importance; but Marquesas Islands. In proportion as the probability is, that the original inthe magnetic poles move in their or- scription being partly decayed, or in bits within the earth, the points where danger of destruction, was from its the magnetic crosses the terrestrial historical importance deemed worthy equator will be found more eastward ; of being renewed in brass at a later
(June, period. The value of the information Kenelworth,Clent, Kenilsham, Winchcontained in it consists in this : that it combe, Hereford, Sutton, Kenchester, is contemporary. Of this there is in- Thorney, St. Alban's, Nottingham, ternal evidence. The writer, if we Warwick, Gloucester, Stamford, Berkemay so call him, speaks in the first ley, Tulbury, Runcorn, Tamworth, person, and gives us the history of his Eddesbury, Sempringham, Lincoln
, family and property. That family was Cuckamsley, Offchurch, Kingsland, of the Royal race of Mercia; and the Kenelworth, Clinton (the same with property comprised most of the ancient Clent before mentioned), from which fortresses arid lands of the Mercian place, and not Glympton in OxfordKings. How much of this property is shire, the noble family of Newcastle still vested in those noble families, takes its name. who are the representatives and de- I will conclude with some remarks scendants of the 'Saxon Kings of Mer- and corrections. For ælc read æc, cia, his Grace the Duke of Newcastle, i. e, eac, also, eke; hebbe is underwho is a lineal descendant of the Rem- stood after forsecan; for nis read nul, BALDS of Clinton, may perhaps be “I will not give,” &c.; Ýr may be capable of ascertaining with the great- considered an interpolation as explanaest facility, by tracing it either to the Kenelms of Leominster, or to the Lory of býð; for mazopine read maRembalds of Clinton, now Clent in gopinc. Staffordshire. With a view to the elu
Yours, &c. cidation of these points, I have ventured to give you, Mr. Urban, my in- DEFENCE OF THE ENGLISH' terpretation of the topography of the
UNIVERSITIES. inscription, which differs in some instances from that of
(Concluded from p. 392.) ent s. 1. E. For example, Deotis
. THE University of Cambridge as if the reading were Đeocirbynig; justice be considered as defective in whereas I consider Đeotisbypiz to be useful learning. Mathematics hare the correct reading, and that Tutbury long been considered the best practical in Staffordshire is intended, — not
system of logic. The art of reasoning Tewksbury in Gloucestershire. Here is essentially necessary in every departwe may remark, Mr. Urban, by the ment and occurrence of life. The'sysway, that in Staffordshire, as well as
tem of education also, which we are in Oxfordshire, and other counties,
vaontingly told “is no suitable prepamany traces are to be found of the ration for the Church, the Bar, or the Celtic worship of Teutates, and other Senate,” has introduced to both Houses Aboriginal deities, adopted by our tinguished members
, whose lives have
of Parliament some of their must disSaxon ancestors also before their conversion to Christianity, which was not
been spent in watching public opia effectually and generally completed at
nions, and in accelerating so early a period as some would per public error. "The present components
progress of public virtue or suade us. Hence Tutsbury and Tetbury or Teut's-Bury, Tetsworth or
members of the University of Cam; Teut's-Worth, Great and Little Tew, bridge, we are further told, are ? &c. &c. But the further illustration
“mere pigmy race,-mere algebraists.
" of this curious subject I leave to Mr. Can Granta's elder sons (I forbear to Bowles; from whom we may soon whom would well bear comparison
mention living characters, many of expect a considerable addition to our topographical stores, in the Parochial with these,) possibly be deemed a History of Bremhill. At present I pigmy race" in literature or science
, shall confine myself to the Leominster whilst she ranks in the number of her Inscription, where the places recorded literary heroes the names of a Bacon, I conceive to be these : Chelmsford,
a Newton, a Milton, a Dryden, a BentPeterborough *, Lichfield, Leicester, ley, or a Porson?
That our “ Senior Wrangler” In the original, Medeswelhamstede, not much more than a inere algebraist, the ancient name of the place ; before as he identifies himself with the UniBurgh, Gildenburgh, and Peterborough, versity, we may, upon his own pre were introduced by the love of innovation. mises, certainly conclude. Having,