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twenty years, therefore, as Baron Humboldt has remarked, and by means of accurate instruments, Russia may make a gigantic progress in the theory of magnetism.

The magnetic action of the globe is not only subject to secular inequalities, but also to diurnal, and even hourly variations. In Europe the variation is most easterly about 8 or 9 o'clock in the morning, and most westerly about 1 or 2 o'clock in the afternoon; the needle then moves eastward till about 8 or 9 o'clock in the evening, when it continues stationary for an hour or two, or makes a slight move ment to the west, so that at about 8 in the evening it is a little farther to the east than it was on the preceding evening.

About the summer solstice, the common diurnal variation is nearly twice as great as about the win ter solstice, being in the first case about 15' and in the last about 7. There is also a regular monthly motion, in virtue of which the needle travels west ward from the summer solstice to the vernal equinox, and eastward from the vernal equinox to the summer solstice.

The diurnal variation increases from the equator to the poles. In Iceland and Greenland the needle attains its most westerly position at from 8 to 10 in the evening; in Europe and the United States from 2 to 3 in the afternoon; in Sumatra about 7 in the morning; in St. Helena about 8. It attains its most easterly position in Europe and North America about 7 or 8 in the morning; in Iceland and Greenland about 9 or 10; in Sumatra about 5 in the evening; and at St. Helena about 6 (or two.) On the northwest coast of America it seems to reach its most westerly position in the forenoon, and its most easterly in the afternoon. These daily oscillations appear to consist of four movements, two directed eastward and two westward.

Besides these regular oscillations, the needle is subject to sudden and extraordinary movements, which baron Humboldt calls magnetic hurricanes, during which the needle traverses, frequently with a shivering motion, or one of several degrees, on both sides of its usual position. These disturbances have often been noticed during the existence of the aurora borealis; and yet captain Foster, when in the arctic regions, was never able to observe the slightest effect produced upon his needles by the most vivid aurora.

The dip of the needle undergoes analogous variations, but a few experiments only seem to have been made on this part of the subject. M. Hansteen, from a series of observations made with a dipping needle by Dollond, found that the dip during the summer was about fifteen minutes greater than what it was during the winter, and about four or five minutes greater in the forenoon than what it was in the afternoon.

Similar variations are observed in the magnetic intensity of the earth. The minimum of the daily variation of intensity is between 10 and 11 in the forenoon, and the maximum between 4 and 5 in the

• Dr. Brewster's Journal of Science, No. X. p. 221.

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afternoon. The intensity is a maximum in December, and a minimum in June. The greatest monthly variation is a maximum in the months of De cember and June, about the time when the earth is in its perihelion or aphelion. It is a minimum near the equinoxes, or when the earth is at its mean distance from the sun. The greatest daily variation is least in the winter, and greatest in the summer. The greatest difference of the annual variation is 0.0359. M. Hansteen has also found that the intensity is decreasing in Europe, and that the decrease is greater in the northerly and easterly parts than in the southern and westerly, an effect obviously produced by the motion of the Siberian pole towards the east.*

During Captain Parry's voyage of 1824 and 1825, a remarkable diurnal variation of the needle was discovered at Port Bowen. The maximum westerly variation was found to occur between 10 a.м. and 1 P.M., and the minimum between 8 P.M. and 2 A.M. The exact time of the maximum as deduced from a mean of 120 days, was 11° 49′ A. M. and that of the minimum 10° 1' P. M. The amount of the diurnal variation was seldom less than 1 or 2°, and it sometimes rose to 5o 6', and even 7°. Captain Parry likewise observed an increase in the magnetic intensity from the morning till the afternoon, and a decrease from the afternoon to the morning.

Various hypotheses have been invented for the purpose of explaining the variations of the needle. Halley at firstt supposed that the earth is a great magnet, having four magnetical poles or points of attraction, two being near each pole, and he endeavoured to fix the position of these poles; but when he considered that no magnet was known to have more than two poles, he abandoned this hypothesis as insufficient to account for the phenomena, he devised the following hypothesis. He conceives the earth to consist of an external shell, which is a magnet, having its two fixed poles distant from the poles of rotation, and he supposes that there is an internal nucleus or inner globe included in the external shell, and separated from it by a fluid. The nucleus, which is also a magnet with two poles, has the same centre and axis of diurnal rotation with the external shell, only this outer sphere having its turbinating motion some small matter either swifter or slower than the external ball. And a very minute difference in length of time, by many repetitions becoming sensible, the internal parts will by degrees recede from the external, and not keeping pace with each other, will appear gradually to move either to the east or west by the dif ference of their motions." This displacement of the internal nucleus, therefore, occasions a change of place in its magnetic poles, and furnishes a rude explanation of the principal facts; but the hypotheIsis is too wild to be admitted into science, and is besides totally insufficient to explain many of the phenomena.

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The general opinion among modern philosophers is, that the magnetism of the earth is related to its

† Phil. Trans. 1683, Vol. X1II. No. 148, p. 208,

Phil. Trans 1692, Vol. XVI. No. 195, p. 563.

temperature, and this opinion may be considered to have received a most striking confirmation, when Dr. Brewster proved by an extensive series of observations, that the temperature of our globe is related to two poles of maximum cold distant from the poles of rotation, and related in position to the magnetic poles.* The two cold poles seem to have different intensities like the magnetic ones, and by supposing them to have a motion of rotation either contemporaneous with, or independent of, the magnetic poles, we may explain those remarkable changes of temperature which have obviously taken place at the same points of our globe. The earth may be conceived to be a great magnet, and then the intensity of its magnetism, at any point, will be inversely as the temperature of that point. Or it may be considered like a piece of soft iron, and as deriving its magnetism from the sun or planets, and in this case the intensity of its magnetism will increase with its temperature. The first of these opinions has been most generally adopted, but the second has acquired great probability from the connexion of the diurnal variation with the motions of the sun, and from the magnetic influence of the solar rays, though this latter influence has again become a matter of well founded doubt.† Mr. Kupffer of Casan has endeavoured, in a very ingenious memoir, to prove that terrestial magnetism re sides on the surface of the globe.

From a more recent comparison of observations, Dr. Brewster has given the following general for mula for the mean temperature at any point of the earth's surface;

T = (tr) (sinë. ♪. sinn. ♪') + in which T = the mean temperature required. t = the maximum equatorial tempera

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tang. cos. M. tang. L, where L is the co-latitude of the pole of maximum cold, I the co-latitude of the place, and M the difference of longitude between the place and the pole.

The above formula is founded on the supposition that the poles of maximum cold are placed in opposite meridians, and have the same temperature, and it gives a series of returning curves of the nature of Lemniscates. The values of t and are already determined with considerable accuracy, t being nearly 82°.8 and from 0° to 34°. The exponent n is nearly, though future observations may induce us to increase or diminish it. Dr. Brewster finds the observations to be the best represented by placing the American pole in Lat. 73°

N. and Lon. 100° W. a little to the east of Cape Walker, and the Asiatic pole in Lat. 739 N. and E. Lon. 80°, between Ubino and Cape Mazol on the gulf of Oby. Hence the two warm meridians will be in West Lon. 10° and East Lon. 170°, the latter passing through Lord Mulgrave's range, and the former between St. Helena and Ascension island. The two cold meridians, or those which pass through the poles of maximum cold, will be in W. Lon. 100° and E. Lon. 80°, the latter passing near Mexico and through Bathurst Island, and the former through Colombo in Ceylon, Berar in Hindostan, and crossing the Oby a little to the west of Narym in Siberia.

Now the same formula will be found, so far as observations at present indicate, to express the magnetic intensity at any point of the earth's sur face, on the supposition of the intensity of the two magnetic poles being equal, calling S the maximum number of seconds in which any number n of oscillations are performed, which takes place at the island of St. Thomas, on the west coast of Africa, and s the minimum number of seconds in which n oscillations are performed, which takes place at the magnetic poles, then the intensity I will be

=

I (S-8) (sin". . sin". ') + 8 and ♪ being determined by the formulæ already given, adopting the position of the poles, as given bine and Hansteen, the values of S and 's will be in the preceding page. According to Captain Saabout 370" and 262" or 263". This formula will give a series of returning curves of the nature of Lemniscates, almost similar to those drawn by Captain Sabine. A correction may be applied to accommodate the formula to the hypothesis of the two poles having different intensities.

It is a curious fact, however, which will require an explanation from any good theory of the magnetic variations, that though according to Hansteen the lines of equal dip and of equal intensity are sensibly parallel in Scotland, yet farther to the east, in Norway and Sweden, the latter tend more to the north, and cut the former; and also under the same line of equal dip the intensity is weaker to the east than to the west. Hence M. Hansteen has found that the pole of inclination or dip, that is, the point to which the lines of equal dip are related, lies in 71° of Lat. and 102° of Lon., while the pole of intensity is in 56° of Lat. and 80° of Lon. west of Paris.‡

We shall now conclude this article with a description of instruments for measuring the variation and dip of the needle, and the intensity of the magnetic force.

Description of instruments for measuring the variation and dip of the needle, and the intensity of the magnetic force.

See our article POLAR REGIONS, Vol. XVII.

These instruments may be thus enumerated: 1. Colonel Beaufoy's variation instrument.

↑ See Dr. Brewster's Journal of Science, No. IV. New Series, April 1830, p. 225. See Dr. Brewster's Journal of Science, No. IV. New Series, April 1830, p. 260.

2. Dollond's variation transit.

3. Dollond's dipping needle.

4. Dollond's diurnal variation instrument. 5. Hansteen's instrument for measuring the magnetic intensity.

6. Mr. Barlow's correcting plate.

1. Colonel Beaufoy's Variation Instrument. The first instrument for measuring the variation of the needle which we propose to describe, is that which was used by Colonel Beaufoy in his numer ous magnetical experiments.

The principal part of this instrument, viz. its needle, with the box and divided arch, to measure the angle of variation, is very similar to the instrument in the possession of the Royal Society; but it is greatly improved by the addition of a small transit telescope, which very readily and accurately determines the true meridian or zero, from which the degree of the variation is to be measured. This addition renders it particularly well adapted for travellers who wish to make accurate observations in different parts of the globe, as it contains every thing necessary for the observations within itself; and as every part admits of adjustment, by reversing, it may be depended upon for accuracy.

A representation of the whole instrument in perspective is given in Plate DXXVIII. Fig. 2, which explains all its parts at once. The box containing the needle is not fixed, as in many compasses, but turns horizontally on the centre, and has an index fastened to it, pointing to a divided arch, on the brass frame on which it turns; and the method of observing is to move the box till a line drawn on it points exactly to the end of the needle, which being done, the angle of variation is shown by the divis. ions on the arch. FF is a mahogany board, which is the support of the whole instrument; it stands on the points of three screws G, H, I, by which it can be levelled. Above this is a flat plate of brass EE, attached to the board by a centre pin, and resting upon three studs, projecting from the board, to ensure its having a perfect bearing, whilst it admits of a small horizontal motion round the centre by means of the screw WX. The plate EE has the divided arch m fixed upon one end of it, and a vernier D traverses against the divisions; it is divided upon a projecting part of a brass plate CC, which moves on the common centre pin of the plate C, and also the needle AAB. The plate CC has two segments of brass at each end of the needle, and these have the centre lines drawn upon them, which are brought to the points of the needle when the observation is made. A light brass box dd, with a glass cover, is fitted over the plate CC to preserve the needle from disturbance from the wind; it also supports a small double microscope M, intended to assist in examining when the index line points exactly to the end of the needle; the stem of the microscope is fitted to a dove-tailed groove, and can be removed to the opposite end of the box at pleasure.

The centre pin of the plates E and C terminates in an extremely fine point, on which the needle is suspended, having an agate cap, B, to diminish the VOL. XVIII. PART I.

friction as much as possible. The needle is of a cylindrical figure, about the five hundredth of an inch in diameter, and ten inches long, and weighs 65 grains, and is terminated by two conical points; the circular enlargement of the centre has a hole through it, for the reception of the brass socket, B, which has the agate cap fixed in it; the needle is, of course, provided with the usual apparatus for lifting it off the point when not in use, to avoid wearing the point of suspension. a, a, are two arms, screwed down upon the plate, carrying the vernier, D; they support a brass frame, b, which has the usual clamp screw, L, and tangent screw, R, the former to fasten it to the arch, m, and the latter to give a slow motion, and adjust the box for the observation.

The transit telescope, O P, is supported over the instrument by two pillars N, N, fixed on the brass plate, E E, and having small frames or boxes, fat the top for the reception of the Y's, in which the pivots of the axis, Q, of the telescope are supported; this axis is conical, and is fixed exactly at right angles to the tube, O P, in the same manner as other transit instruments.

On the extremity of one of the pivots of the axis, a small divided circle, R, is fixed, and has an arm at the opposite side turning about on the centre, and provided with verniers to read the divisions on the circle; to this arm a small level, S, is attached; the whole forming a flying index to set the telescope at any required altitude, by setting the index at the proper division, and then moving the telescope till the bubble of the level, S, shows the index to be horizontal; the eye-piece, h, of the telescope has a small dovetail in it to admit a dark glass for observation of the sun; g, are the screws for adjusting the wires in the eye-piece as usual; the separate figure, TV, is a level to adjust the in

strument.

Manner of adjusting the instrument previous to observation. First place the feet, k, l, of the level, upon the brass plate EE, in different directions, and bring it level by the screws, H, I, K; then apply the level upon the two pivots of the transit, the covers of the boxes, f, opening with hinges for this purpose, and the bottom of the feet, k, l, have notches to rest upon the pivots; if the axis does not prove level, one of the Y's must be elevated or depressed, by a screw in the farther pillar, N, until the level stands horizontally, and, reversing it end for end, proves every thing to be correct.

Now set the vernier, D, at zero, and put an additional object glass, over the glass, P, of the telescope; and in this state the marks near AA, against which the needle reads, can be seen through it when directed to them; this proves the zero of the division to be exactly in the plane of the telescope motion; a small screw near Q will rectify it, if necessary, by moving the Y a small quantity in the box, f; the transit may be reversed in its Y's to rectify every thing.

The telescope is now to be adjusted to the meridian by the transit of the stars in the usual manner, the screws, W, X, being used to turn the whole instrument round when requisite; but after these

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observations have been once made in a satisfactory manner, distant marks should be set up, both north and south, and these will give the means of adjusting it at once, and in the day time.

When the instrument is properly placed, the taking of the observation is extremely simple. The needle being suffered to settle, the box is turned about on its centre till the mark comes very near the point of the needle; the clamp screw, L, is then fastened, and the screw k is employed to make the coincidence exact, in which the microscope, M, greatly assists the eye. The vernier, D, now shows the angle of variation. It is proper, after the needle has been once observed, to attract it with a piece of iron, and cause it to make a slight vibration. When it settles again, it will not in all cases come to the same point, because of the friction of the suspending point; it is therefore advisable to make this trial three or four times, and take the mean of the whole.

The instrument is placed on a pillar, in the open air, free from any iron, and is adjusted before any observation; the vernier is divided to half minutes, and if the meridian be not at a considerable distance, an advantage will be gained by diminishing the aperture of the object glass by a pasteboard cap. I have found the needles should not exceed the weight mentioned, 65 grains.

It may be worth while to remark, that this instrument will serve as a portable transit instrument for naval officers to examine the rate of their chronometers while on shore.

Colonel Beaufoy afterwards made some changes upon this instrument. The base of it, instead of resting upon a piece of mahogany, was placed on a brass triangular stand, through the angles of which were inserted the mill-headed screws for levelling it. By this improvement the instrument when once levelled retains its horizontal position, the wooden stand being liable to warp. The object glass was also made so as to be capable of adjustment for different distances. One of the needles is a very slender parallellopipedon, and weighs 50 grains. The other is of a cylindrical form, terminating at each end with a cone, and weighs 63 grains. The increase of weight in the needle is owing to the new agates which have been put in.

2. Variation Transit, as constructed by Dollond. The variation transit, represented in Plate DXXIX. Fig. 1, is constructed with the requisite apparatus and adjustments for observing in A R, and when correctly adjusted to the meridian, the needle will show the true variation. For the proof of this, there is a lens fitted into a cap or cover, represented at a, which is placed before the object glass of the telescope, whereby it is converted into a transit microscope, the focus of the lens being suited to the distance of the needle, and the divisions of the compass circle; and its centre being made to correspond correctly with the centre of the object glass of the telescope, the collimation of the wires continuing the same. By this method the correct place of the divisions, as well as of the needle, may be readily ascertained, and the extreme as well as the diurnal variation truly determined.

This instrument is also useful for taking altitudes, equal altitudes, and as a theodilite.

3. Dipping Needle, as constructed by Dollond. The dipping needle as represented in fig. 2, is of the plainest construction. Similar instruments are sometimes made with an apparatus for discharging the needle, when used for determining the magnetic force, and for raising and lowering the axis of the needle, with other trifling additions. The instrument here described is furnished with adjustable agate planes for the pivots of the needle to rest upon; and for the purpose of proving the horizontality of these planes, there is a small plate a attached to it, the under surface of the plate being ground truly flat, and the level adjusted parallel to that surface.

4

The needles are shown at 1, 2, 3, 4, 5. The first is upon the plan recommended by Mayer in his Treatise on the Magnetic Dip, and has the centre of gravity at a distance from the centre of motion. This is produced by the small ball at the side. An account of this needle was also published by captain Sabine. The second figure shows the edge of 1 and 3 with the axis in its place. 3 is the plain needle, which at this period appears to be the most approved, as being the least encumbered, and of course the simplest in its use and reductions. was recommended by Dollond upon the principle of doing away any indirect polarity that might be occasioned from the pores of the steel or inequality of magnetising the needle. It is formed with a cube in the middle, from which extends two cones; the cube having two holes at right angles to each other, into which the axis is placed. By this contrivance, each face of the needle may be changed. The theory of this needle is correct, but the use of it is tedious. An account of its performance was published by captain Sabine.

The fifth is the form of needle recommended by captain Kater, which for lightness, and the strength arising from its figure, appears to have given very satisfactory results, where the magnetic power has been feeble. It has been generally used by captain Foster in his experiments.

4. Diurnal Variation Instrument as constructed by Dollond.

Fig. 3, is the section of an instrument for determining the diurnal variation of the magnetic needle. It is made of mahogany and ivory, for the purpose of avoiding the attraction that has been suspected in metallic instruments. In this figure, the silk fibre with the needle at its lower end, and the ball or counterpoise at the other end, passed over the pulley, is shown in the middle of the square box between the two microscopes, a, a; each of these microscopes is furnished with two cross wires; these are brought to correspond with the line on the ends of the needle, by means of the nut, b, which also, and at the same time, moves the frame c, to which the verniers d, e, Fig. 4, are attached. These verniers are read off on the arcs f, g, and give the angle of the diurnal variation. Each end of the needle must be taken to correct excentricity. The needles used with this instrument are represented

at Figs 5, and 6; it is also furnished with a piece of brass similar in form, and of the same weight as the needles, for the purpose of detecting the twist of the silk fibre. This instrument may also be used for the horizontal magnetical force, in which case it would require to have a contrivance applied for discharging the needle at the required angle. In general the magnetic force apparatus has been fitted up separately. The diurnal part of the instrument was made by order of the government for captain Foster, who published an account of the variations observed with it, in the Philosophical Transactions. Magnetic force instruments were also furnished by Mr Dollond for his expedition.

5. Professor Hansteen's Instrument for determining the Magnetic intensity.

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This instrument is represented in Plate DXXIX, Fig. 7. It consists of a well hardened magnetic cylinder of steel, a, b, about 24 inches long, and ths of an inch thick. The needle is suspended by a fibre c d, from the cod of the silk worm in a square box MN made of mahogany. The cover MN of the box consists of three parts, of which the two outermost ones, M, N, have glass windows which may be drawn out. The middle part P is of mahogany, and in the centre of it is screwed a hollow wooden tube T along the axis of which the pipe passes which suspends the cylindrical needle. The box rests on three screws S, S, S, by which it is set horizontally either on a smooth surface or on a tripod stand when it is carried about in travelling. On the bottom of the box a divided arc, or a whole circle graduated on paper, is fixed for the purpose of reading off the oscillations of the cylindrical needle a b. The free extremity of the suspending fibre is fixed to a back at T. The needle is drawn out of the magnetic meridian by applying to the end of the box a little iron rod, which being held vertically, has in its lower part a north pole. On removing it the needle begins to oscillate, and when the elongation is 20, the observer determines by means of a chronometer the duration of 300 oscillations, which gives a measure of the magnetic force, as already fully explained in our article MAGNETISM.

6. Mr Barlow's Correcting Plate. When we consider the great quantity of iron which necessarily enters into the construction of a ship and its appendages-with the guns, anchors, &c. we are astonished that the probable disturbance of the needle on ship board from these causes had not long ago led to a full examination of all the circumstances of this action. It appears, however, that the effect itself was only first observed by Mr Wales in one of the voyages of captain Cook, and even then, although the effect was observed, the cause was not looked after-it was merely stated as a curious circumstance, that the needle had a different bearing in different parts of the vessel, and under different courses-the subject at that time therefore attracted little attention. The next reference to the local attraction of vessels, and in which the cause is clearly pointed out, is found in

Walker's Treatise on Magnetism, published in 1794. It is contained in a report from Mr Downie, master in H. M. S. Glory, where he says, "I am convinced that the quantity and vicinity of iron in most ships have an effect in attracting the needle; for it is found by experience, that the needle will not always point in the same direction, when placed in different parts of the ship; also it is rarely found that two ships steering the same course by their respective compasses, when compared on board the same ship, will agree exactly with each other.”

Soon after this report, the action of the iron of the vessel on the compass was more minutely noticed by captain Flinders, who was the first to trace its connexions with the dip of the needle, and through whose perseverance some attention was paid by the government to the subject, several experiments having been made, by order of the admiralty, on various ships at the Nore, by which the general fact was established. The subject, however, seems to have been again lost sight of, till Mr Bain, a master in the navy, published his valuable little "Treatise on the Variations of the Compass," in which the fatal consequences attending this source of error are so clearly pointed out, as to strike the most indifferent readers. About this time the first of our arctic voyages of discovery was in contemplation, and the local attraction of the vessels was one of the objects to which the attention of the officers was particularly directed by the admiralty.

The results of these experiments are to be found in the accounts of the first voyages of captain Ross, captain Parry, &c. as also by captain Sabine, in the Phil. Trans. Part I. 1819, and from which it will be seen, that the amount of error due to this cause in high northern latitudes is such, as to render the needle entirely useless, or worse than useless, in the navigation of those seas.

The existence of a serious evil being thus fully established, the next object was to devise some principle of calculation, or other method, of determining its amount in all cases, in order to its being allowed for in the course of the vessel. Amongst those who engaged in this interesting pursuit, Mr Barlow of the Royal Military Academy, Woolwich, was the most successful. In order to proceed upon some certain grounds, he first undertook a long series of experiments, with a view of reducing the action of iron on a magnetic needle to mathematical laws, and in which he was completely success ful. By this means, the local attraction of ships became a matter of calculation, but still, however, requiring as one of the data the dip of the needle, which can never be very accurately known on ship board. The same philosopher, however, at the close of his experiments, fortunately, made another very important discovery, as it related to the subject of his investigation, and, at the same time, highly interesting as a philosophical fact, viz. that the attracting power of iron is not resident in the mass, but on the surface of the iron; so that, in fact, a hollow shell of iron of about 4 lbs. weight, acts as strongly on the needle, at the same distance, as a solid iron ball of 200 lbs. This being estab

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