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more particularly in the island of Ceylon, where it is called by the natives tournamal.

The first account we have had, of late years at least, of this extraordinary stone, was in the History of the Royal Academy of Sciences of Paris, for the year 1717; where we are told, that Mr. Lemery exhibited a stone, which he said was not common, and came from Ceylon. This stone attracted and repelled little light bodies, such as ashes, filings of iron, bits of paper, and such like. The publisher of that history then proceeds to give some reasons for these phenomena. Linneus, in his preface to the Flora Zeylanica, mentions this stone under the name of lapis electricus; and takes notice of Lemery's experimentsbefore-mentioned. Notwithstanding this, no further mention was made of this stone, and its effects, till very lately. The Duke de Noya, in his letter to M, de Buffon, informs us, that when at Naples in the year 1743, the late Count Pichetti, secretary to the king, assured him, that during his stay at Constantinople, he had seen a small stone, called a tourmaline, which attracted and repelled ashes. This account the Duke de Noya had quite forgot; but, being last year in Holland, he saw and purchased two of these stones, which are there called aschentrikker. The making experiments with these called to his remembrance what formerly had been told him by Count Pichetti. With these stones he made, in company with Messrs. Daubenton and Adanson, a great number of experiments, of which the duke has favoured the public with a particular



In the year 1757, there were two accounts published on this subject: the one is a memoir of M. Æpinus, read to the Royal Academy at Berlin, intitled, De Quibusdam Experimentis Electricis Notabilioribus. The other is a treatise in quarto, printed at Rostock, intitled, Disputatio de Electricitatibus Contrariis. Auctore Joanne Carolo Wilke. Since which time, Dr. Heberden, having procured some of these stones from Holland, a great number and variety of experiments with them have been made here, particularly by Mr. Wilson; an account of which he has very lately communicated to the Royal Society.

XXXVIII. An Attempt to Account for the Regular Diurnal Variation of the Horizontal Magnetic Needle; and also for its Irregular Variation at the Time of an Aurora Borealis. By John Canton, M. A., F. R. S. p. 398. The late Mr. George Graham made many observations on the diurnal variation of the magnetic needle,, in the years 1722 and 1723: but declared himself ignorant of the cause of that variation, in N° 383 of the Philosophical Trans. actions, where many of those observations are to be found. About the year 1750, Mr. Wargentin, secretary of the Royal Academy of Sciences in Sweden, took notice both of the regular diurnal variation of the needle, and also of its

being disturbed at the time of an aurora borealis, as recorded in the 47th volume of the Philosophical Transactions; but is silent as to the cause. Mr. C. had no opportunity of making observations of this sort himself, till the latter end of the year 1756; but, after that time, he made near 4000, with an excellent variation-compass, of about 9 inches in diameter. The number of days on which these observations were taken, was 603; and the diurnal variation on 574 of them was regular; that is, the absolute variation of the needle westward, was increasing from about 8 or 9 o'clock in the morning till about 1 or 2 in the afternoon, when the needle became stationary for some time; after that, the absolute variation westward was decreasing, and the needle came back again to its former situation, or near it in the night, or by the next morning. The diurnal variation is irregular when the needle moves slowly eastward in the latter part of the morning, or westward in the latter part of the afternoon; also when it moves much either way after night, or suddenly both ways within a short time. These irregularities seldom happen more than once or twice in a month, and are always accompanied (so far as he had been able to observe) with an aurora borealis. Thus having explained what he means by the regular and irregular diurnal variation, and showed, that this variation is generally regular; he then, in the first place, endeavours to account experimentally for the regular variation; then offers a conjecture concerning the cause of the regular variation and, lastly, attempts to make it appear probable, that the aurora borealis arises from the same cause.

The attractive power of the magnet, (whether natural or artificial) will decrease while the magnet is heating, and increase while it is cooling; as will appear by the following experiments.

Exper. 1. About E. N. E. from a compass a little more than 3 inches in diameter, he placed a small magnet 2 inches long, half an inch broad, and of an inch thick, parallel to the magnetic meridian; and at such a distance, that the power of the south end of the magnet was but just sufficient to keep the north end of the needle to the N. E. point, or to 45 degrees. The magnet. being covered by a brass weight of 16 oz. about 2 oz. of boiling water was poured into it, by which means the magnet was gradually heating for 7 or 8 minutes; and during that time the needle moved about three quarters of a degree westward, and became stationary at 44°; in 9 minutes more it came back a quarter of a degree, or to 44°; but was some hours before it gained its former situation, and stood at 45o. N. B. The greater the power of the same magnet, the more it will lose in a given degree of heat.

•Exper. 2. On each side of the compass, and parallel to the magnetic meridian, he placed a strong inagnet of the size above-mentioned; so that the south ends of both the magnets acted equally on the north end of the needle, and kept it

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in the magnetic meridian; but if either of the magnets was removed, the needle was attracted by the other, so as to stand at 45 degrees. The magnets were both covered with brass weights of 16 oz. each. Into the eastern weight he poured about 2 oz. of boiling water; and the needle in one minute moved half a degree, and continued moving westward for about 7 minutes, when it arrived at 20. It was then stationary for some time; but, in 24 minutes from the beginning, it came back to 2o, and in 50 minutes to 2o4. He then filled the western weight with boiling water, and in one minute the needle came back to 1°; in 6 minutes more it stood half a degree eastward; and after that, in about 40 minutes, it returned to the magnetic north, or its first situation.

It is evident that the magnetic parts of the earth in the north on the east side, and the magnetic parts of the earth in the north on the west side of the magnetic meridian, equally attract the north end of the needle. If then the eastern magnetic parts are heated faster by the sun in the morning than the western, the needle will move westward, and the absolute variation will increase; when the attracting parts of the earth on each side the magnetic meridian have their heat increasing equally the needle will be stationary, and the absolute variation will then be greatest; but when the western magnetic parts are either heating faster, or cooling slower than the eastern, the needle will move eastward, or the absolute variation will decrease; and when the eastern and western magnetic parts are cooling equally fast the needle will again be stationary, and the absolute variation will then be least. This may be still further illustrated by placing the compass and two magnets, as in the last experiment, behind a screen near the middle of the day in summer; then, if the screen be so moved that the sun may shine only on the eastern magnet, the needle will sensibly vary in its direction, and move towards the west; and if the eastern magnet be shaded while the sun shines on the western, the needle will move the contrary way. By this theory, the diurnal variation in the summer ought to exceed that in the winter; and accordingly it is found by observation that the diurnal variation in the months of June and July is almost double that of December and January.

The irregular diurnal variation must arise from some other cause than that of heat communicated by the sun; and here Mr. C. has recourse to subterranean heat, which is generated without any regularity as to time, and which will, when it happens in the north, affect the attractive power of the magnetic parts of the earth on the north end of the needle. Dr. Hales has a good observation on this heat, in the Appendix to the second volume of his Statical Essays, viz." that the warmth of the earth, at some depth under ground, has an influence in promoting a thaw as well as the change of the weather from a freezing to a thawing state, is manifest from this observation; viz. Nov. 29, 1731,

a little snow having fallen in the night, it was by 11 the next morning, mostly melted away on the surface of the earth, except in several places in Bushy-Park where there were drains dug, and covered with earth, where the snow continued to lie, whether those drains were full of water or dry; as also where elm-pipes lay under ground; a plain proof that these drains intercepted the warmth of the earth from ascending from greater depths below them; for the snow lay where the drain had more than 4 feet depth of earth over it. It continued also to lie on thatch, tiles, and the tops of walls."

That the air nearest the earth will be most warmed by its heat is obvious; and this has frequently been taken notice of in the morning, before day, by means of thermometers at different distances from the ground, by Dr. Miles, at Tooting in Surrey; and is mentioned in p. 526, of the 48th volume of the Philosophical Transactions.

The aurora borealis, which happens at the time the needle is disturbed by the heat of the earth, is supposed to be the electricity of the heated air above it; and this will appear chiefly in the northern regions, as the alteration in the heat of the air in those parts will be greatest. This hypothesis will not seem improbable, if it be considered that electricity is now known to be the cause of thunder and lightning; that it has been extracted from the air at the time of an aurora borealis: that the inhabitants of the northern countries observe the aurora to be remarkably strong, when a sudden thaw happens after severe cold weather; and that the curious in these matters are now acquainted with a substance, that will, without friction, both emit and absorb the electrical fluid, only by the increase or diminution of its heat: for if the tourmalin be placed on a plane piece of heated glass, or metal, so that each side of it, by being perpendicular to the surface of the heating body, may be equally heated; it will, while heating, have the electricity of one of its sides positive, and that of the other negative; this will likewise be the case when it is taken out of boiling water, and suffered to cool; but the side that was positive while it was heating, will be negative while it is cooling, and the side that was negative, will be positive.

For the sake of those who may be desirous of examining the diurnal variations of the needle very minutely, Mr. C. annexed a complete year's observations; and deduced from the regular variations during that time, the mean diurnal variation belonging to each month whence it appears that the diurnal variation increases from January to June, and decreases from June to December.

The mean diurnal Variation for each

Month in the Year 1759.




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7 8

8 58

11 17

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12 26

13 O

13 21

13 14

12 19

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11 43 10 36


8 9

6 58

XXXIX. On the Sections of a Solid, hitherto not considered by Geometers. By
Wm. Brakenridge, D.D., F.R.S. p. 446.
On an imaginary solid that never can occur, or be of

real use. any

XL. A Letter to the Hon. J. Th. Klein, Secretary to the City of Dantzic, from Mr. Peter Collinson, F.R.S., concerning the Migration of Swallows. p. 459. Mr. C. dissents from Mr. K. in an article he takes great pains to establish; which is, that swallows are not birds of passage; but at the time of their disappearing retire under water, and live therein all the winter. This Mr. C. cannot comprehend, being so contrary to nature and reason; for as they cannot live in that state without some degree of breathing, this requires the circulation of the blood, however weak and languid. Now as respiration is absolutely necessary for circulation, how is it possible to be carried on for so many months under water without the risk of suffocation? Besides, if so remarkable a change was intended, the great wisdom of the Almighty Creator would undoubtedly be seen in some particular contrivance, in the structure of the organs of the heart of this bird, to enable it to undergo so very remarkable a change of elements.

An easy experiment may throw some light on this doubtful affair. At the time of their going away, take a swallow, and confine it in a tub under water: if it remains there for a week or two alive, without any remarkable inconvenience, then there may be some probability of its continuing so many months in that state. The conclusions that are drawn from some of the tribe of insects subsisting under water, are far from being conclusive to found an analogy on; as insects differ from other animals in so many particulars, that very little or nothing can be concluded or inferred of the one from what we observe in the other.

Towards the end of September, the swallows assemble on the reeds in the islands in our river Thames, and have no doubt so done for ages past; and yet Mr. C. never heard or read of any fisherman or other person that has ever found in the winter months a swallow under water in a torpid living state; for if such a marvellous thing had ever happened, it would have been soon communicated to the public. Besides, as these islands of reeds and willows are annually cut. down for several uses, and yet not a swallow has been discovered in his aquatic abode; and considering the multitudes seen on these reeds and willows in the autumn; if they took their winter's residence under water, it is most reasonable to think, in a river so frequented, and in so long a course of years, some would have been found in that situation. Another circumstance is, that in great towns remote from water, where rivers and reeds are not near, it is frequently observed that a little before the swallows depart, they every morning early gather together 3 I


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