that there has been no alteration in the variation of the compass in Jamaica for
these 140 years.
We know that both on the west and east side of Jamaica, the
variation has changed prodigiously within that time: and one can conceive no
reason why this should not have happened also in Jamaica.

riation.

The diurnal variation of the needle was discovered by Mr. George Graham, Diurnal Va in 1724, and it was explained soon after in a satisfactory manner, by Canton. Van Swinden and Coulomb have also given us explanations of it, but not so satisfactory as that of Canton.

The dip of the needle was first observed by Norman, and little addition has Dip. since been made to his hypothesis and collection of facts. From the united observations which have been made in different times and places, it appears that the dip is subject to much less variation than the declination. Some persons, indeed, have supposed that the dip does not vary at all; but the very careful observations of Gilpin, lately published, demonstrate, that at London it is at present diminishing. The magnetic equator, as laid down by Wilke and Lemonnier, is nearly the very same which has been lately deduced from the observations of Humboldt, by Biot. To Biot we are likewise indebted for a mathematical hypothesis, (though not quite correct), and for a very good table of observations.

It was Graham that first thought of measuring the magnetic intensity by the vibrations of the needle; a method afterwards used by Coulomb, and considered by many as invented by him. From the observations made by Humboldt, and Gay Lussac, in this manner, Biot has deduced the variation of intensity in different latitudes.

The magnetic laws were first generalized and explained by Dr. Gilbert, whose book on magnetism, published in 1600, is one of the finest exemples of inductive philosophy that has ever been presented to the world. It is the more remarkable, because it preceded the Novum Organum of Bacon, in which the inductive method of pholosophizing was first explained. Dr. Brook Taylor, and afterwards Muschenbroeck, attempted without success to determine by experiment the rate at which the magnetic attractions and repulsions vary. This rate was successfully investigated by the subsequent experiments of Lambert, Robison, and Coulomb. The nature of magnetic curves was first satisfactorily explained by Lambert, Robison, and Playfair.

The first attempt to make artificial magnets, as far as I know, was by Sellers, Artificial magin 1667.* There is said to be a paper on this subject by Reaumur in the nets. Mcmoires of the French Academy for the year 1723. In the year 1730, Servington Savery succeeded in making them, and in showing them to be preferable to loadstones. Dr. Gowan Knight soon after made very powerful artifi

Phil Trans. Vol. II. p. 478.

Papers in the
Transactions,

cial magnets. He concealed his method; but after his death it was published, and said to consist in making up a paste of elutriated iron filings and linseed oil. His experiments were published in 1744. Mr. Mitchell, of Cambridge, published an improved method in 1750, and Mr. Canton another in 1751. Antheaume's method was published, in the year 1760, in a separate treatise, but it had appeared before in the Memoires of the French Academy for 1753. Æpinus and Coulomb proposed good methods of making artificial magnets, but later than any of those which have been already enumerated.

From the preceding statements it appears that almost all the discoveries relative to magnetism have been made in England. The most distinguished names as discoverers are those of Gilbert, Norman, Gellibrand, Halley, Savery, Graham, Knight, Mitchell, and Canton. Lambert, Epinus, and Coulomb, hold the most distinguished place among foreigners.

The papers on magnetism, in the Philosophical Transactions, amount to seventy-eight; but of these twenty-one may be passed over as trifling. The following is a sketch of the topics treated of in the remaining papers, omitting those that have been already noticed.

1. Colepress found that heat weakened the magnetic energy. Phil. Trans. 1667. Vol. II. p. 502.

2. Mr. Bond published a table of the variation of the needles, at London, for the next 50 years. But his hypothesis turned out inaccurate. Ibid. p. 789. 3. Dr. Halley attempted to explain the declination of the needle by the hypothesis of the earth being a great magnet with four poles, two in the north and two in the south. The change of declination he accounted for, by supposing the earth a hollow sphere enclosing another sphere at its centre. Each sphere has two poles, and the internal has a slow rotatory motion west. This ingenious hypothesis never gained much ground, and at present, we believe, has no supporter. Phil. Trans. 1683. Vol. XIII. p. 208; and 1692. Vol. XVII.

p. 563.

4. An iron bar held in the proper direction becomes a magnet. Phil. Trans. 1685. Vol. XV. p. 1213.

5. Twisting a wire destroys its magnetism. Splitting a wire often inverts the poles or destroys the polarity. The result is affected by the side of the wire laid uppermost. Derham. Phil. Trans. 1705. Vol. XXIV. p. 2136.

d2

1

d3

6. Dr. Brook Taylor made experiments to ascertain the law of magnetic attraction. At small distances it was nearly as 1 ; but at greater distances as or more. He gave four poles to a wire by touching it at one end, or at various parts. Phil. Trans. 1715. Vol. XXIX. p. 294; and 1721. Vol. XXXI. p. 204. 7. Mr. Graham found the dip at London, in 1723, to be 74° 40'. In the year 1747, he found it 73° 30'. In the year 1723, the needle made 100 vibra

tions in 5′ 35" at London. Phil. Trans. 1725. Vol. XXXIII. p. 332; and 1748. Vol. XLV. p. 279.

8. Mr. Savery describes his method of making artificial magnets. Phil. Trans. 1730. Vol. XXXVI. p. 295.

9. Captain Middleton observed that needles would not traverse in the neighbourhood of ice, and that cold had the same effect; but they recovered their qualities when heated. Phil. Trans. 1738. Vol. XL. p. 310.

10. The magnetic experiments of Dr. Gowan Knight are circumstantially related. The only novelties which they exhibit are the great magnetic power which the magnets employed possessed, and the facility with which a powerful magnet reverses the poles of a smaller one. Phil. Trans. 1744. Vol. XLIII. p. 161, 361; and Vol. XLIV. p. 656.

11. The diurnal variation of the needle was first observed by Graham, then by Celsius, then by Wargentine, Canton, &c. Phil. Trans. 1751. Vol. XLVII. p. 126.

12. The diurnal variation is least in December, and greatest in June at London. It is westward from eight in the morning till one or two o'clock, then returns east again till the evening. Mr. Canton accounts for it in a satisfactory manner by showing that heat diminishes the magnetic power of a needle. The diurnal variation becomes irregular when an aurora borealis appears. The following table gives us the mean diurnal variation for each month of the year 1759, ás observed by Mr. Canton.

Thus we see that it is greatest in June, and least in December. Phil. Trans. 1759. Vol. LI. p. 398.

13. Various tables of the variation occur in the Philosophical Transactions. The following, not reckoning Halley's which has been already quoted, are the most remarkable of these. MONTAINE. Phil. Trans. 1766, Vol. LVI. p. 216. Ross. Ibid. p. 218. WALES. Ibid. 1769. Vol. LIX. p. 467. Сook. Ibid. 1771. Vol. LXI. p. 422.

14. In the year 1772, the dip, at London, appears from the experiments of Mr. Nairne to have been about 72° 18'. Phil. Trans. 1772. Vol. LXII. p. 476. 15. Mr. Hutchins gives us the dip in the year 1774, in different places. The following are the principal:

16. A particular description of Dr. Gowan Knight's great magnetical magazine is given, and likewise an account of the method which he employed in making his artificial magnets. Phil. Trans. 1776. Vol. LXVI. p. 591; and 1779. Vol. LXIX. p. 51.

17. Cavallo found that brass becomes magnetic by hammering, though it contains no sensible quantity of iron. The magnetism is destroyed by heating the brass red hot, or by partially melting it. Phil. Trans. 1786. Vol. LXXVI. p. 62.

18. Cavallo shows that the magnetism of bodies may be best tried by putting them on the surface of clean mercury, and then bringing a magnet near them. Phil. Trans. 1787. Vol. LXXVII. p. 6.

19. Mr. Bennet proposes the suspension of a fine sewing needle by a spider's thread in the glass of a gold leaf electrometer, as an excellent means of discovering very minute quantities of magnetic attraction. Phil. Trans. 1792. Vol. LXXXII. p. 81.

20. Mr. Macdonald gives us an account of the diurnal variation of the needle in Sumatra. It was less than Mr. Canton observed it in Londoù, not exceeding 2' or 3'; but the whole variation of the needle at Fort Marlborough, in 1794, was only 1° 8′ east. Mr. Macdonald considers the diurnal variation as a confirmation of Halley's hypothesis of the four magnetic poles of the earth. Phil. Trans. 1796. Vol. LXXXVI. p. 340.

8'

21. The same gentleman has given us the diurnal variation at St. Helena. He found it to be 3′ 55′′ in November when the whole variation was 15° 48′ 341′′ west. Phil. Trans. 1798. Vol. LXXXVIII. p. 397.

BOOK IV.

OF CHEMISTRY.

WE shall divide this Book into three Chapters. In the first we shall treat of Chemistry, strictly so called; in the second, of Meteorology; and in the third, of those Chemical Arts and Manufactures which are described in the Philosophical Transactions.

CHAP. I.

OF CHEMISTRY.

science.

THE origin and primitive meaning of the word Chemistry are not accurately Origin of the known. The science itself is of modern date, and scarcely preceded the origin of the Royal Society. It sprung from a sect of fanatics, who occupied them- The alchyselves assiduously in endeavouring to convert the baser metals into gold and mists. silver. These men, in the course of the numberless experiments which they made upon the metals, and upon other bodies capable of bearing the heat of their fires, or of altering the appearance of their metals, discovered a variety of powerful agents hitherto unknown; and ascertained many curious and important properties of the substances on which they experimented. These they registered in their writings, in a language at once pedantic and obscure, and thickly scattered with the most childish and ridiculous absurdities. Some of these alchymists, as they were called, being medical men, began to think of applying some of the preparations which had been thus discovered in medicine. The effects of these new medicines, particularly the preparations of antimony, the first chemical medicines brought into general use in Europe, were so powerful, as not only to draw the attention of the medical faculty, but even to occupy the deliberations of whole kingdoms. Paracelsus, who carried these chemical Paracelsus. medicines to an extravagant length, and who boasted that he was in possession of one, capable of curing all diseases, and of prolonging life to an indefinite