plate to such a depth, that their axis, or centre, precisely corresponds with the centre of the hinges; hence the middle of the extremity of each bar is at the same elevation, and at the same distance from the needle in every position of the moveable limb. To give firmness to the instrument in making experiments, the table is fixed by the feet to a mass of lead, of seven or eight pounds weight. By means of this plate of lead, which has a screw at each corner, the whole apparatus is readily put into a horizontal position.

With this apparatus, Mr Scoresby made a series of experiments, which are fully detailed in the Transactions of the Royal Society of Edinburgh, Vol. IX., and of which the following are the principal results.

1. Iron bars become magnetical by position, excepting when placed in the plane of the magnetic equator; the upper end, as regards the position of the magnetic equator, becoming a south pole, and the lower extremity a north pole.

2. No attraction or repulsion appears between a magnetized needle and iron-bars; the latter being free from permanent magnetism, whenever the iron is in the plane of the magnetic equator; consequently by measuring the angle of no-attraction, in a bar placed north and south, we discover the magnetic dip.

3. Before a magnet can attract iron, that is totally free from both permanent magnetism and that of position, it, infuses into the iron a magnetism of contrary polarity to that of the attracting pole.

4. A bar of soft iron, held in any position, except in the plane of the magnetic equator, may be rendered magnetical by a blow with a hammer, or other hard substance; in such cases, the magnetism of position seems to be fixed in it, so as to give it a permanent polarity.

5. An iron-bar, with permanent polarity, when placed anywhere in the plane of the magnetic equator, may be deprived of its magnetism by a blow.

6. Iron is rendered magnetical if scowered or filed, bent or twisted, when in the position of the magnetic axis, or near this position; the upper end becoming a south pole, and the lower end a north pole; but the magnetism is destroyed by the same

means, if the bar be held in the plane of the magnetic equa

tor.

7. Iron heated to redness, and quenched in water, in a vertical position, becomes magnetic; the upper end gaining south polarity, and the lower end north.

8. Hot iron receives more magnetism of position than the same when cold.

9. A bar-magnet, if hammered when in a vertical position, or in the position of the magnetic axis, has it power increased, if the south pole be upward, and loses some of its magnetism if the north end be upward.

10. A bar of soft steel, without magnetic virtue, has its magnetism of position fixed in ft, by hammering it when in a vertical position; and loses its magnetism by being struck when in the plane of the magnetic equator.

11. An electrical discharge, made to pass through a bar of iron, devoid of magnetism, when nearly in the position of the magnetic axis, renders the bar magnetic'; the upper end becoming a south pole, and the lower end a north pole; but the discharge does not produce any polarity, if the iron be placed in the plane of the magnetic equator. The effects appear to be the same, whether the discharge be made on the lower or upper end of the bar, or whether it is passed longitudinally or transversely through the iron.

12. A bar of iron possessing some magnetism, has its polarity diminished, destroyed, or inverted, if an electric discharge be passed through it, when it is nearly in the position of the magnetic axis, provided the south pole of the bar be downward; while its magnetism is weakened or destroyed, if it receive the shock when in the plane of the magnetic equator.

13. Iron is rendered magnetical, if a stream of the electric fluid be passed through it, when it is in a position nearly corresponding with that of the magnetic axis; but no effect is produced, when the iron is in the plane of the magnetic equator.

ART. XXIV.-Notice respecting Professor HANSTEEN's Chart of the Variation and Dip of the Needle.

THE chart of the variation and dip of the magnetic needle which accompanies this Number, and which is marked PLATE IV. was drawn by Professor Hansteen from the numerous tables of the declination and inclination of the needle which are given in his work on the Magnetism of the Earth, of which we have already presented an analysis to our readers. This chart occupies two separate plates in Hansteen's Magnetic Atlas; but we have thought it preferable to unite both in one, in order that the relation of the lines of Variation and Dip may be at once visible to the eye.

The Lines of Equal Variation are projected from observations reduced to the year 1787, whereas the lines of equal dip are projected from observations reduced to the year 1780.

In the original chart published by Hansteen, he makes the magnetic equator cut the terrestrial equator only twice, viz. in 108° of West, and 21° of East Longitude *; but in his letter to our correspondent M. Rumker, of which we have already availed ourselves in this Number, he states, that his own chart is in this respect erroneous, and that the Magnetic Equator actually crosses the terrestrial equator FOUR times, viz, in 25° of East, and 108°, 125°, and 170° of West Longitude,—a correction which we have carefully made in the accompanying chart, altering, at the same time, the adjacent lines of 10°, 20° and 30° of North and South Dip, which must necessarily follow the inflexions of the Magnetic Equator.

The Western Line of No Variation, which is more strongly marked than the rest, and passes along the Atlantic, and to the west of Hudson's Bay, corresponds, very nearly, with the same line on the chart after Churchman, which we have already men

In the chart of the variation and dip, principally from Churchman, and reduced to 1794, published by Dr Thomas Young, and forming Plate 48d of his Elements of Natural Philosophy, vol. i. the Magnetic Equator cuts the real Equator only twice, in 170° of West and 15° of East Longitude,

tioned; but the Eastern line differs in a very remarkable manner from the same line in Churchman.

In Hansteen's chart, the Eastern line of no variation passes through New Holland to Archangel, after making numerous inflexions among the Indian Islands, and through the continent of Asia; while Churchman gives it nearly a rectilineal course from New Holland to a little beyond Enisesk in Siberia, where he makes it terminate. In the chart of Hansteen, the lines of eastern variation from 0° to 10°, partake of course of the sinuosities of the line of no variation; but though they resume a more uniform course to the west, yet in the Pacific Ocean they form a series of returning curves of the shape of a heart, which have no resemblance to those in Churchman's chart.

In examining the different groups of the Variation Lines, the reader will observe that they are represented on the engraving by three different kinds of lines. The lines marked thus, continuously black, are the lines which are best determined. Those marked thus, ... have an infe rior degree of evidence; while those marked by a single dotted line thus, .............. are merely interpolated by estimation. The same remarks apply to the Lines of Dip, where, however, only the continuous and the dotted lines are used. The Lines of 65° and 75° of North Dip, for example, are entirely interpolated; and also the Asiatic portion of the lines of 60° and 70° of North Dip, and the Atlantic portion of 60° of South Dip.

In the present chart, we have added the new discoveries of Captain Parry, and New Shetland; and also one or two points of variation in Baffin's Bay, together with the Poles of maximum Cold, which we have recently deduced from a comparison of meteorological observations *.

EDINBURGH,
March 1. 1821.

}

D. B.

See Transactions of the Royal Society of Edinburgh, vol. ix. p. 214.; and the present volume of this Journal. p. 193, 194.

ART. XXV.-Account of a Remarkable Shower of Hail which fell in Orkney on the 24th of July 1818. By PATRICK NEILL, F. R. S. Edin. F. L. S. & Sec. Wern. Soc. *

THE morning of the 24th of July 1818, was, in Orkney,

clear and warm, with a slight air of wind at due south. About midday the atmosphere became overclouded. Between twelve and one o'clock, thunder and lightning began; and after these had continued with little intermission for about an hour and a half, the Reverend Mr Taylor of Stronsa observed a very dense jet black cloud, apparently rising from the sea, at the distance of about five or six miles. It then seemed of no great dimensions; but its magnitude was gradually developed, as it approached steadily, and apparently with increasing velocity, from the southward, in a direct line toward the centre of the island. It now assumed a dismally ominous aspect, and occasioned a considerable degree of darkness. The lightning became proportionally more vivid, and the peals of thunder more tremendous. Mr Taylor remarked one flash of lightning to be not only brighter than the rest, but to exert a more extensive influence on the cloud, which seemed as if cleft asunder, and presented a momentary opening of the prospect between the Mainland of Orkney and the Island of Stronsa. The thun der-bolt on this occasion seemed to strike the surface of Stronsa Frith in the manner of a solid body dashing into the sea.

Richard Caithness, who possesses the farm of Hunday in the island of Stronsa, was engaged in the making of kelp on the shore, when he perceived the cloud advancing fast towards his own farm-steading. He immediately hurried home. At this time the wind began to rise; the surface of the sea was greatly ruffled; and darkness like that of night threatened to come on. Just as he reached his house, the cloud overtook him.

The lightning was now instantaneously followed by

Abridged from the Transactions of the Royal Society of Edinburgh, vol. ix p 187., to which the reader is referred for a fuller account of this very remarkable phenomenon. Mr Neill's paper is illustrated with a map shewing the track of the hail shower.