a white earth procured from the mother-liquors of nitre, or sea-salt, by precipitation with a fixed alkali, and afterwards washing of the salt with water. See Chemistry, n° 37.

Magnesia alba is a good absorbent; and undoubtedly ly to be preferred to crab's eyes, on account of its purgative quality when united with an acid, which the other has not. It has been deemed hurtful in bilious habits where there is a disposition in the stomach contrary to acidity. This, however, according to Mr Henry, is doubtful; and where putrid bile is to be corrected, he thinks good purposes may be answered by taking magnesia with an acid in a state of effervescence; as the fixed air, thus extricated, will correct the putridity of the contents of the intestines, while they are at the same time evacuated downwards. He is also of opinion, that in cutaneous diseases it may enter the circulation in form of a neutral salt, and, by acting as a diaphoretic and diuretic, prove an excellent alternative.

For some medical purposes, magnesia is used in a calcined state; in which case it is deprived of its fixed air, and then it proves nearly as aperient as a double quantity of magnesia in its uncalcined state. Mr Henry is of opinion, that it may be useful in distensions of the bowels arising from flatus; that it may be successfully employed as a cathartic with patients labouring under the stone, who are using the lixivium saponacum; and that, joined with warm aromatics, it may be of service in correcting the great flatulency which so much afflicts people of a gouty disposition. From several experiments made by the same author, it also appears that magnesia has a considerable antiseptic power. The like virtue he attributes to all kinds of tinctuous powders; from whence he concludes, that medicines of this kind are by no means improper in fevers of a putrefactive type; that where bile is suspected to be the cause of any putrid disease, those antiseptics should be prescribed which particularly impede its corruption; that, as calcined magnesia is a more powerful antiseptic than most other absorbents, it merits a preference to these; and that where an acid cacochymy prevails, magnesia or other absorbents, taken immediately before or after meal-time, may, by increasing the putrefactive fermentation of animal-food, be of very great service. He hath also found, that magnesia hath a power of promoting the solution of gelatinous gums in water; and thus we have an elegant and easy method of preparing aqueous tinctures from these substances. Such tinctures, however, are calculated only for extemporaneous prescription, as most of them deposit a sediment when they have been kept a week or two.

Magnesia, (anc. geog.), a maritime district of Thessaly, lying between the south part of the Sinus Thermaticus and the Pagasaeus to the south, and to the east of the Pelasgiotis. Magnetes, the people. Magnesius and Magnesius, the epithet, (Horace).

Magnesia ad Sipylum, (anc. geog.), a town of Ionia, on the Meander, to distinguish it from another Magnesia at the foot of mount Sipylus. The former was one of the three towns given to Themistocles by Artaxerxes, with these words, "to furnish his table with bread." A colony from the Magnesia of Thessaly, (Pliny); from Delphi, (Athenaeus); from Lacedemon, (Velleius); distant 15 miles from Ephesus to the east, (Pliny). It did not stand immediately on the Meander, being nearer the river Lethæus, which runs into the Meander, (Strabo). It is sometimes mentioned without its distinguishing surname, as being more considerable than the other Magnesia, which is scarce ever without its surname ad Sipylum. Magnetes, the people.

Magnesia ad Sipylum, (anc. geog.), anciently Tantalis, the residence of Tantalus, and capital of Maeonia, where now stands the lake Sale. A town of Lydia, at the foot of mount Sipylus, to the east of the Hermus; adjudged free under the Romans; destroyed by earthquakes. (Strabo).

Magnet, Magnes, the Leadstone; a sort of ferruginous stone, in weight and colour resembling iron ore, though somewhat harder and more heavy; endued with divers extraordinary properties, attractive, directive, inclinatory, &c. See Magnetism.

The magnet is also called Lapis Heracleus, from Heraclea, a city of Magnesia, a port of the ancient Lydia, where it is said to have been first found, and from which it is usually supposed to have taken its name. Though others derive the word from a shepherd named Magnes, who first discovered it with the iron of his crook on mount Ida. It is also called lapis nauticus, by reason of its use in navigation; and feriteries, from its attracting iron, which the Greeks call sidus.

The magnet is usually found in iron mines, and sometimes in very large pieces, half magnet, half iron. Its colour is different, according to the different countries it is brought from. Norman observes, that the best are those brought from China and Bengal, which are of an irony or languine colour; those of Arabia are reddish; those of Macedonia blackish; and those of Hungary, Germany, England, &c. the colour of unwrought iron. Neither its figure nor bulk are determined, but it is found of all forms and sizes.

The ancients reckoned five kinds of magnets different in colour and virtue: the Ethiopic, Magnesian, Boeotic, Alexandrian, and Natolian. They also took it to be male and female; but the chief use they made of it was in medicine; especially for the cure of burns, and defluxions on the eyes.—The moderns, more happy, take it to conduct them in their voyages. See Navigation.

The most distinguishing properties of the magnet are, That it attracts iron, and that it points to the poles of the world; and in other circumstances also dips or inclines to a point beneath the horizon, directly under the pole; and that it communicates these properties, by touch, to iron. On which foundation are built the mariner's needles, both horizontal, and inclinatory or dipping needles.

Attractive Power of the Magnet was known to the ancients, and is mentioned even by Plato and Euripides, who call it the Herculean stone, because it commands iron, which subdues every thing else: but the knowledge of its directive power, whereby it disposes its poles along the meridian of every place, and occasions needles, pieces of iron, &c. touched with it, to point nearly north and south, is of a much later date; though the exact time of its discovery, and the discoverer himself, are yet in the dark. The first tidings we hear of it is in 1260, when Marco Polo the Venetian is said by some to have introduced the mariners compass; tho' not as an invention of his own, but as derived ved from the Chinese, who are said to have had the use of it long before; tho' some imagine that the Chinese rather borrowed it from the Europeans.

Flavio de Gioa, a Neapolitan, who lived in the 13th century, is the person usually supposed to have the best title to the discovery; and yet Sir G. Wheeler mentions, that he had seen a book of astronomy much older, which supposed the use of the needle; though not as applied to the uses of navigation, but of astronomy. And in Guyot de Provins, an old French poet, who wrote about the year 1180, there is an express mention made of the loadstone and the compass; and their use in navigation obliquely hinted at.

The Variation of the Magnet, or its declination from the pole, was first discovered by Seb. Cabot, a Venetian, in 1500; and the variation of that variation, by Mr Gellibrand, an Englishman, about the year 1625. See Variation.

Lastly, the dip or inclination of the needle, when at liberty to play vertically, to a point beneath the horizon, was first discovered by another of our countrymen, Mr R. Norman, about the year 1576. See the article Dipping Needle.

Phenomena of the Magnet. 1° In every magnet there are two poles, one whereof points northwards, the other southwards; and if the magnet be divided into ever so many pieces, the two poles will be found in each piece. The poles of a magnet are found by holding a very fine short needle over it; for where the poles are, the needle will stand upright, but nowhere else. 2° These poles in different parts of the globe, are differently inclined towards a point under the horizon. 3° These poles, though contrary to one another, do help mutually towards the magnet's attraction and suspension of iron. 4° If two magnets be spherical, one will turn or conform itself to the other, so as either of them would do to the earth; and after they have so conformed or turned themselves, they endeavour to approach or join each other; but if placed in a contrary position, they avoid each other. 5° If a magnet be cut through the axis, the parts or segments of the stone, which before were joined, will now avoid and fly each other. 6° If the magnet be cut by a section perpendicular to its axis, the two points, which before were conjoined, will become contrary poles; one in one, the other in the other segment. 7° Iron receives virtue from the magnet by application to it, or barely from an approach near it, though it do not touch it; and the iron receives this virtue variously, according to the parts of the stone it is made to touch, or even approach to. 8° If an oblong piece of iron be any how applied to the stone, it receives virtue from it only as to its length. 9° The magnet loses none of its own virtue by communicating any to the iron; and this virtue it can communicate to the iron very speedily: though the longer the iron touches or joins the stone, the longer will its communicated virtue hold; and a better magnet will communicate more of it, and sooner, than one not so good. 10° Steel receives virtue from the magnet better than iron. 11° A needle touched by a magnet will turn its ends the same way towards the poles of the world, as the magnet itself does. 12° Neither loadstone nor needles touched by it do conform their poles exactly to those of the world, but have usually some variation from them; and this variation is different in divers places, and at divers times in the same place. 13° A loadstone will take up much more iron when armed or capped than it can alone. And though an iron ring or key be suspended by the loadstone, yet the magnetic particles do not hinder that ring or key from turning round any way, either to the right or left. A loadstone is said to be armed, when its poles are surrounded with plates of steel. To determine the quality of steel to be applied, try the magnet with several steel bars; and the greatest weight it takes up, with a bar on, is to be the weight of its armur. 14° The force of a loadstone may be variously increased or lessened by the various application of iron, or another loadstone, to it. 15° A strong magnet at the least distance from a lesser or weaker, cannot draw to it a piece of iron adhering actually to such lesser or weaker stone; but if it come to touch it, it can draw it from the other: but a weaker magnet, or even a little piece of iron, can draw away or separate a piece of iron contiguous to a greater or stronger loadstone. 16° In these northern parts of the world, the south pole of a loadstone will raise up more iron than the north pole. 17° A plate of iron only, but no other body interposed, can impede the operation of the loadstone, either as to its attractive or directive quality. Mr Boyle found it true in glasses sealed hermetically; and glass is a body as impervious as most are to any effluvia. 18° The power or virtue of a loadstone may be impaired by lying long in a wrong position, as also by rust, wet, &c. and may be quite destroyed by fire. 19° A piece of iron wire well touched, will, upon being bent round in a ring, or coiled round on a stick, &c. generally, quite lose its directive virtue, but always have it much diminished; and yet if the whole length of the wire were not entirely bent, so that the ends of it, though but for the length of one tenth of an inch, were left straight, the virtue will not be destroyed in those parts; though it will in all the rest. This was first observed by Grimaldi and de la Hire; and is confirmed by the experiments of Mr Derham; who adds further, that though coiling or bending the wire as above, would always destroy its virtue by day, yet it would not do it in the evening. 20° This sphere of the activity of magnets is greater and less at different times: in particular, that referred in the repository of the royal society will keep a key or other body suspended to another, sometimes, at the height of eight or ten feet; and at others, not above four feet. To which we may add, that the variation of the magnetic needle from the meridian, varies at various times of the day; as appears from some experiments of Mr Graham, and likewise from observations made during one of Capt Cook's voyages. See Variation. 21° By twisting a piece of wire touched with a magnet, its virtue is exceedingly diminished; and sometimes so disordered and confused, that in some parts it will attract, and in others repel; and even, in some places, one side of the wire seems to be attracted, and the other side repelled, by one and the same pole of the stone. 22° A piece of wire that has been touched, being split, or cleft in two, the poles are sometimes changed, as in a cleft magnet; the north becoming the south, and the south the north; and yet... sometimes one half of the wire will retain its former poles, and the other half will have them changed. To which it may be added, that laying one or other side of the half uppermost, causes a great alteration in its tendency or aversion to the poles of the magnet.

23°. A wire being touched from end to end with the same pole of the magnet, the end whereat you begin will always turn contrary to the pole which touched it; if it be again touched the same way with the other pole of the magnet, it will then be turned the contrary way.

24°. If a piece of wire be touched in the middle with only one pole of the magnet, without moving it backwards or forwards; in that place will be the pole of the wire, and the two ends will be the other pole.

25°. If a magnet be heated red hot, and again cooled either with its south pole towards the north in a horizontal position, or with its south pole downwards in a perpendicular position, its poles will be changed.

26°. Mr Boyle (to whom we are indebted for the following magnetic phenomena) found he could presently change the poles of a small fragment of a loadstone, by applying them to the opposite vigorous ones of a large magnet.

27°. Hard iron tools well tempered, when heated by a brisk attrition, as filing, turning, &c. will attract thin filings or chips of iron, steel, &c. and hence we observe files, punches, augers, &c. to have a small degree of magnetic virtue.

28°. The iron bars of windows, &c. which have a long time stood in an erect position, grow permanently magnetic; the lower ends of such bars being the north-pole, and the upper the southern.

29°. A bar of iron that has not stood long in an erect posture, if it be only held perpendicularly, will become magnetic, and its lower end the north pole, as appears from its attracting the south pole of a needle; but then this virtue is transient, and by inverting the bar the poles will shift their places.

In order therefore to render the quality permanent in an iron bar, it must continue a long time in a proper position. But the fire will produce the effect in a short time; for as it will immediately deprive a loadstone of its attractive virtue; so it soon gives a verticity to a bar of iron, if being heated red hot, it be cooled in an erect posture, or directly north and south. Nay, tongs and fire-forks, by being often heated and set to cool again in a posture nearly erect, have gained this magnetic property. Sometimes iron bars, by long standing in a perpendicular position, have acquired the magnetic virtue in a surprising degree. A bar about 10 feet long, and three inches thick, supporting the summer-beam of a room, was able to turn the needle at eight or ten feet distance, and exceeded a loadstone of three and an half pounds weight. From the middle point upwards it was a north pole, and downwards a south pole; and Mr Martin mentions a bar, which had been the beam of a large steel-yard, that had several poles in it.

30°. Mr Boyle found, that by heating a piece of English ooker red-hot, and placing it to cool in a proper posture, it manifestly acquired a magnetic virtue. And an excellent magnet of the same ingenious gentleman's having lain near a year in an inconvenient posture, had its virtue surprisingly impaired, as if it had been by fire.

31°. A needle well touched, it is known, will point north and south; if it have one contrary touch of the same stone, it will be deprived of its faculty; and by another such touch will have its poles quite changed.

32°. If a bar of iron have gained a verticity by being heated red-hot and cooled again, north and south, and then hammered at the two ends; its virtue will be destroyed by two or three smart blows on the middle.

33°. By drawing the back of a knife, or long piece of steel-wire, &c. leisurely over the pole of a loadstone; carrying the motion from the middle of the stone to the pole, the knife or wire will accordingly attract one end of a needle; but if the knife or wire be passed from the farthest pole to the middle of the stone, it will repel that end of the needle which in the other case it attracts.

34°. Either a magnet or a piece of iron being laid on a piece of cork, so as to swim freely in water; it will be found, that whichever of the two is held in the hand, the other will be drawn to it; so that iron attracts the magnet as much as it is attracted by it; action and re-action being always equal. In this experiment, if the magnet be set afloat, it will direct its two poles to the poles of the world.

35°. A knife, &c. touched with a magnet, acquires a greater or less degree of virtue, according to the part it is touched on. It receives the strongest touch, when it is drawn leisurely from the handle towards the point over one of the poles: And if the same knife thus touched, and thus in possession of a strong attractive power, be retouched in a contrary direction, viz. by drawing it from the point towards the handle over the same pole, it immediately loses all its virtue.

36°. A magnet acts with equal force in vacuo and in the open air.

37°. The smallest magnets have generally the greatest power in proportion to their bulk. A large magnet will seldom take up above three or four times its own weight; whereas a small one will frequently take up more than ten times its weight. A magnet worn by Sir Isaac Newton in a ring, and which weighed only three grains, would take up 746 grains, or almost 250 times its own weight. A magnetic bar made by Mr Canton, according to the method described in the next article, and which weighed 10 ounces 12 pennyweights, took up something more than 79 ounces; and a flat semicircular steel magnet that weighed an ounce and 13 penny-weights took up an iron wedge of 90 ounces.

Artificial Magnet, is a term usually applied to steel bars impregnated with the virtues of the natural magnet or loadstone; and are much more common, as well as more convenient for use, than the others.

The late Dr Godwin Knight possessed a surprising skill in magnetism, being able to communicate an extraordinary degree of attractive or repulsive virtue, and to alter or reverse the poles at pleasure; but as he refused to discover his methods upon any terms whatever, (even, as he said, though he should receive in return as many guineas as he could carry,) these curious and valuable secrets have died with him. In the 69th volume of the Philosophical Transactions, however, Mr Benjamin Wilson hath given a process which at least discovers one of the leading principles of Dr Knight's art, and may perhaps be a means of discovering the whole to those who shall be less reserved. The doctor's process, according to Mr Wilson, was as follows. Having provided himself with a great quantity of clean iron-filings, he put them into a large tub that was more than one third filled with clean clean water; he then, with great labour, worked the tub to and fro for many hours together, that the friction between the grains of iron by this treatment might break off such smaller parts as would remain suspended in the water for a time. The obtaining of these very small particles in sufficient quantity seemed to him to be one of the principal deliderata in the experiment. The water being by this treatment rendered very muddy, he poured the same into a clean iron vessel, leaving the filings behind; and when the water had stood long enough to become clear, he poured it out carefully, without disturbing such of the sediment as still remained, which now appeared reduced almost to impalpable powder. This powder was afterwards removed into another vessel in order to dry it; but as he had not obtained a proper quantity thereof by this one step, he was obliged to repeat the process many times. Having at last procured enough of this very fine powder, the next thing was to make a paste of it, and that with some vehicle which would contain a considerable quantity of the phlogistic principle: for this purpose, he had recourse to linseed oil in preference to all other fluids. With these two ingredients only he made a stiff paste, and took particular care to knead it well before he moulded it into convenient shapes. Sometimes, while the paste continued in its soft state, he would put the impression of a seal upon the several pieces; one of which is in the British museum. This paste was then put upon wood, and sometimes on tiles, in order to bake or dry it before a moderate fire, at about the distance of a foot or thereabouts. He found that a moderate fire was most proper, because a greater degree of heat made the composition frequently crack in many places. The time required for the baking or drying of this paste was generally about five or six hours before it attained a sufficient degree of hardness.

When that was done, and the several baked pieces were become cold, he gave them their magnetic virtue in any direction he pleased, by placing them between the extreme ends of his large magazine of artificial magnets for a few seconds or more, as he saw occasion. By this method the virtue they acquired was such, that, when any of those pieces were held between two of his best ten-guinea bars, with its poles purposely inverted, it immediately of itself turned about to recover its natural direction, which the force of those very powerful bars was not sufficient to counteract.

As to the method of making artificial magnets of steel, none hath succeeded in it better than Mr. Canton, whose process is as follows.

Procure a dozen of bars; six of soft steel, each three inches long, one quarter of an inch broad, and one twentieth of an inch thick; with two pieces of iron, each half the length of one of the bars, but of the same breadth and thickness; also six pieces of hard steel, each five inches and a half long, half an inch broad, and three-twentieths of an inch thick; with two pieces of iron of half the length, but the whole breadth and thickness of one of the hard bars; and let all the bars be marked with a line quite round them at one end. Then take an iron poker and tongs, (fig. 1.) or two bars of iron, the larger they are and the longer they have been used, the better; and fixing the poker upright between the knees, hold to it, near the top, one of the soft bars, having its marked end downward, by a piece of sewing silk, which must be pulled tight by the left hand, that the bar may not slide: then grasping the tongs with the right hand, a little below the middle, and holding them nearly in a vertical position, let the bar be stroked by the lower end from the bottom to the top, about ten times on each side, which will give it a magnetic power sufficient to lift a small key at the marked end: which end, if the bar was suspended on a point, would turn towards the north, and is therefore called the north pole; and the unmarked end is, for the same reason, called the south pole.

Four of the soft bars being impregnated after this manner, lay the two (fig. 2.) parallel to each other, at the distance of one fourth of an inch, between the two pieces of iron belonging to them, a north and a south pole against each piece of iron: then take two of the four bars already made magnetical, and place them together so as to make a double bar in thickness, the north pole of one even with the south pole of the other; and the remaining two being put to these, one on each side, so as to have two north and two south poles together; separate the north from the south poles at one end by a large pin, and place them perpendicularly with that end downward on the middle of one of the parallel bars, the two north poles towards its south, and the two south poles towards its north end: slide them backward and forward three or four times the whole length of the bar, and removing them from the middle of this, place them on the middle of the other bar as before directed, and go over that in the same manner: then turn both the bars the other side upwards, and repeat the former operation: this being done, take the two from between the pieces of iron; and, placing the two outermost of the touching bars in their rooms, let the other two be the outermost of the four to touch these with; and this process being repeated till each pair of bars have been touched three or four times over, which will give them a considerable magnetic power, put the half-dozen together after the manner of the four, (fig. 3.) and touch them with two pair of the hard bars placed between their irons, at the distance of about half an inch from each other; then lay the soft bars aside, and with the four-hard ones let the other two be impregnated (fig. 4.), holding the touching bars apart at the lower end near two tenths of an inch; to which distance let them be separated after they are set on the parallel bar, and brought together again before they are taken off: this being oblitered, proceed according to the method described above, till each pair have been touched two or three times over. But as this vertical way of touching a bar will not give it quite so much of the magnetic virtue as it will receive, let each pair be now touched once or twice over in their parallel position between the irons (fig. 5.), with two of the bars held horizontally, or nearly so, by drawing at the same time the north of one from the middle over the south end, and the south of the other from the middle over the north end of a parallel bar; then bringing them to the middle again, without touching the parallel bar, give three or four of these horizontal brakes to each side. The horizontal touch, after the vertical, will make the bars as strong as they possibly can be made, as appears by their not receiving any additional strength, when the vertical touch is given by a great number of bars, and Magnetism.

The quality or constitution of a body, and its pores, whereby it is rendered magnetic, or a magnet. See Magnet.

Magnetism is found to be a transient power, capable of being produced and destroyed again.

Sect. I. The Laws of Magnetism.

The laws of magnetism are laid down by Mr Whiston in the following propositions:

1°. The loadstone has both an attractive and a directive power united together, whereby iron touched by it has only the former; i.e., the magnet not only attracts needles or filings of steel, but directs them to certain different angles with respect to its own surface and axis; whereas iron touched with it, does little or nothing more than attract them; still suffering them to lie along or stand perpendicular to its surface and edges in all places, without any such special direction.

2°. Neither the strongest nor the large magnets give a better directive touch to needles than those of a less size or virtue: to which it may be added, that whereas there are two qualities in all magnets, an attractive and a directive one; neither of them depend on, or are any argument of, the strength of the other.

3°. The attractive power of magnets, and of iron, will greatly increase or diminish the weight of needles on the balance; nay, will overcome that weight, and sustain other additional weights too: while the directive power has much smaller effect. Gassendus indeed, as well as Mersennus and Dr. Gilbert, maintain it has none at all: but mistakenly; for Mr Whiston found, from repeated trials on large needles, that after the touch they weighed less than before. One of 4584½ grains lost 2½ grains by the touch; and another of 6572½ grains weight, no less than 14 grams.

4°. It is probable that iron consists almost wholly of the attractive particles; and the magnet, of the attractive and directive together: mixed, probably, together with other heterogeneous matter; as having never been purged by the fire, which iron has; and hence may arise the reason why iron, after it has been touched, will lift up much greater weights than the loadstone that touched it.

5°. The quantity and direction of magnetic powers, communicated to needles, is not properly, after such communication, owing to the magnet which gave the touch; but to the goodness of the steel which receives it, and to the strength and position of the terrestrial loadstone, whose influence alone those needles are afterwards subject to, and directed by: so that all such needles, if good, move with the same strength and point to the same angle; what loadstone forever (provided it be good) they were excited by. Nor does the touch seem to do much more in magnetic, than attrition in electrical cases; i.e., it serves to rub off some obstructing particles, that adhere to the surface of the steel, and open the pores of the bodies touched, and so make way for the entrance and exit of such effluvia as occasion or assist the powers we are speaking of. Hence Mr Whiston takes occasion to observe, that the directive power of the loadstone seems to be mechanical; and to be derived from magnetic effluvia, circulating continually round it.

6°. The absolute attractive power of different armed loadstones, is, ceteris paribus, according to the quantity, not of their diameters or solidities, but of the surfaces of the loadstones, or in a duplicate proportion of their diameters.

7°. The power of good magnets unarmed, not sensibly different in strength, similar in figure and position, but unequal in magnitude, is sometimes a little greater, sometimes a little less, than in the proportion of their similar diameters.

8°. The loadstone attracts needles that have been touched, and others that have not been touched, with equal force, at distances unequal, viz. where the distances are to one another as 5 to 2.

9°. Both poles of a loadstone equally attract needles, till they be thoroughly touched: then it is, and then only, that one pole begins to attract one end, and repel the other; though the repelling pole will still attract upon contact, nay at very small distances, notwithstanding.

10°. The attractive power of loadstones, in their similar position to, but different distances from magnetic needles, is in the sesquiduplicate proportion of the distances of their surfaces from their needles reciprocally, or as the mean proportionals between the squares and the cubes of those distances reciprocally, or as the square roots of the fifth powers of those distances reciprocally. Thus the magnetic power of attraction, at twice the distance from the surface of the loadstone, is between a fifth and sixth part of that power at the first distance. At thrice the distance, the power is between the 15th and 16th parts; at four times the distance, the power 32 times as small; and at five times the distance, 88 times as small. Where it is to be noted, that the distances are not taken, as in the law of gravity, from the centre, but from the surface; all experience affording us, that the magnetic power resides chiefly, if not wholly, in the surface of the loadstones and iron, without any particular relation to centre at all. The proportion here laid down was determined by Mr Whiston, from a great number of experiments of Mr Haukbee, Dr Brook Taylor, and himself; measuring the force by the chords of those arcs by which the magnet at several distances draws the needle out of its natural direction, to which chords (as he has demonstrated) it is ever proportional. The numbers in some of their most accurate trials he gives us in the following table, setting down half the chords, or the sines of half those arches of declination, as the true measures of the power of magnetism.

| Distances | Degrees of inclination in inches | Sines of inclination | Rat. of sq. arcs. | Rat. of cub. arcs. | |-----------|---------------------------------|---------------------|------------------|-------------------| | 20 | 2 | 175 | 466 | | | 14 3/8 | 4 | 349 | 216 | | | 13 3/8 | 6 | 523 | 170 | | | 12 3/8 | 8 | 697 | 138 | | | 11 3/8 | 10 | 871 | 105 | | | 10 3/8 | 12 | 1045 | 87 | | | 9 3/8 | 14 | 1219 | 70 | |

11°. An inclinatory, or dipping-needle, of six inches radius, and of a prismatic or cylindric figure, when it oscillates along the magnetic meridian, performs here every mean vibration in about 6" or 360"; and every small oscillation in about 5 1/2", or 330"; and the same kind of needle, four feet long, makes every mean oscillation in about 24", and every small one in about 22".

12°. The entire power of magnetism in this country, as it affects needles a foot long, is to that of gravity nearly as 1 to 300; and as it affects needles four feet long, as 1 to 600.

13°. The quantity of magnetic power accelerating the same dipping-needle, as it oscillates in different vertical planes, is ever as the cosines of the angles made by those planes, and the magnetic meridian, taken on the horizon.

Thus if we would estimate the quantity of forces in the horizontal and vertical situations of needles at London; we shall find that the latter, in needles a foot long, is to the entire force along the magnetic meridian as 96 to 100; and in needles four feet long, as 9667 to 10,000; whereas, in the former, the entire force in needles a foot long, is as 28 to 100; and in those four feet long, as 2560 to 10,000.

Whence it follows, that the power by which horizontal needles are governed in these parts of the world is but one quarter of the power by which the dipping-needle is moved.

Hence also, since the horizontal needle is moved only by a part of the power which moves the dipping-needle; and that it only points to a certain place in the horizon, because that place is the nearest its original tendency of any its situation will allow it to tend to; whenever the dipping-needle stands exactly perpendicular to the horizon, the horizontal needle will not respect one point of the compass more than another, but will wheel about every way uncertainly.

14°. The time of oscillation and vibration, both in dipping and horizontal needles equally good, is as their length directly; and the actual velocity of their points along their arcs always equal. Hence magnetic needles are, ceteris paribus, still better the longer they are; and that in the same proportion with their lengths.

The law of magnetic attraction is not yet ascertained. Sir Isaac Newton supposes it to decrease nearly in the triplicate ratio of the distance: but Dr Helsham, trying the experiment by his loadstone, found it to be as the squares of the distances inversely; and Mr Martin affirms us, that the power of his loadstone decreases in a different manner from either, it being in the sesquiplicate ratio of the distances inversely. For exactness, he made a square bar of iron just a quarter of an inch thick, and then provided three pieces of wood of the same form and thickness exactly; then poising the loadstone very nicely at the end of a balance, which would turn with less than a grain, he placed under it the iron with first one piece of wood, then two pieces, and lastly all three pieces upon it: by which means the steel points of the pole were kept at 1/4, 1/2, 3/4, of an inch from the iron; and in those distances the weights put into the opposite scale, to raise the loadstone from the wood, which is touched while the beam was horizontal, were as follows:

| Grains | Rat. of sq. | Rat. of cub. | S. rat. | |--------|------------|-------------|--------| | 156 | 156 | 56 | 156 | | 50 | 39 | 19 | 56 | | 28 | 17 | 6 | 30 |

Whence it appears, that the number of grains to counteract the power of the loadstone in these distances, approach very near, and are almost the same with those which are in the sesquiplicate ratio, but are widely different from those which are in the duplicate ratio; and this experiment Mr Martin tried several times for each distance, with scarce any variation in the success.

The ingenious Muschenbroek has, with indefatigable pains and application, made experiments of the attractions and repulsions of loadstones in respect to iron and to each other; but could never find any regular proportion in the increase of attraction in their approach to, or decrease of attraction in their recess from, one another: only that the force of the magnetic virtue did increase in the approach to, and diminish in the recess from the stone, but not exactly as the distance, nor as the square or cube of the distance, nor as the square or cube of the distance reciprocally, nor in any proportion reducible to numbers; and therefore he conjectures, that the repulsions and attractions disturb one another, so as to confound the proportion.

Sect. II. Of the Causes of Magnetism.

With respect to the causes of magnetism, nothing hath hitherto appeared that can be called a satisfactory solution of its phenomena. It is certain indeed, that both natural and artificial electricity will give polarity to needles, and even reverse their poles; but though from this it may appear probable that the electric fluid is also the cause of magnetism, yet in what manner the fluid acts while producing the magnetic phenomena seems to be totally unknown. All that hath been discovered with regard to this matter is, that a shock from a jar moderately charged, sent from end to end through a fine needle, will give it it a polarity. If the needle is reversed, and a similar shock sent through it the contrary way, the polarity will be destroyed; a third shock will reverse the poles; and the same thing is done by a second shock, if much stronger than the first. If the shock is sent through the sides of the needle, its ends will point east and west; the reason of which is, that one side of it becomes a north, and the other a south pole. Most authors agree, that the end at which the electric blast enters becomes the north-pole; but, from some experiments, this seems very doubtful. The degree of magnetic virtue which electricity can communicate, is very far from being ascertained.

The direction of the magnetic effluvia is thought to be shown by the following experiment. Let AB, CD (fig. 7.) be the poles of a magnet. Round every side lightly throw steel filings, on a sheet of white paper; the particles of the filings will be so affected by the effluvia the stone, as to show the course they take every way. In the middle of each pole, between A B and C D, they appear to proceed in lines nearly straight; towards the ends, they are more and more curved; till at last the lines from both sides, coinciding with each other, form numberless curves round the stone, which are nearly of a circular figure, as in the plate. Hence it is inferred that the magnetic effluvia, issuing from one pole, circulates to the other.

**Sect. III. Entertaining Experiments.**

Construction of the Magnetic Perspective-glass.] Provide an ivory tube, about two inches and a half long, and of the form expressed in fig. 8. The sides of this tube must be thin enough to admit a considerable quantity of light. It is to open at one end with a screw: at that end there must be placed an eye-glass A of about two inches focus, and at the other end any glass you please. Have a small magnetic needle, like that placed on a compass. It must be strongly touched, and so placed at the bottom of the tube that it may turn freely round. It is to be fixed on the centre of a small ivory circle C, of the thickness of a counter, which is placed on the object-glass D, and painted black on the side next it. This circle must be kept fast by a circular rim of palisado, that the needle may not rise off its pivot, after the same manner as in the compass. This tube will thus become a compass, sufficiently transparent to show the motions of the needle. The eye-glass serves more clearly to distinguish the direction of the needle; and the glass at the other end, merely to give the tube the appearance of a common perspective. It will appear from the laws of magnetism already laid down, that the needle in this tube, when placed over, and at a small distance from, a magnet, or any machine in which it is contained, will necessarily place itself in a position directed by that magnet, and consequently show where the north and south pole of it is placed; the north end of the needle constantly pointing to the south end of the magnet. This effect will take place, though the magnet be inclosed in a case of wood, or even metal, as the magnetic effluvia penetrates all bodies. You must observe, however, that the attracting magnet must not be very far distant from the needle, especially if it be small, as in that case its influence extends but to a short distance. This tube may be differently constructed, by placing the needle in a perpendicular direction, on a small axis of iron, on which it must turn quite freely, between two small plates of brass placed on each side the tube: the two ends of the needle should be in exact equilibrium. The north and south ends of this needle will, in like manner, be attracted by the south and north ends of the magnetic bar. The former construction, however, appears preferable, as it is more easily excited, and the situation of the needle much more easily distinguished.

**Exper. I. The communicative crown.**

Take a crown-piece, and bore a hole in the side of it; in which place a piece of wire, or a large needle, well polished, and strongly touched with a magnet. Then close the hole with a small piece of pewter, that it may not be perceived. Now the needle in the magnetic perspective before described, when it is brought near to this piece of money, will fix itself in a direction correspondent to the wire or needle in that piece. Desire any person to lend you a crown-piece, which you dextrously change for one that you have prepared as above. Then give the latter piece to another person, and leave him at liberty either to put it privately in a snuff-box, or not; he is then to place the box on a table, and you are to tell him, by means of your glas, if the crown is or is not in the box. Then bringing your perspective close to the box, you will know, by the motion of the needle, whether it be there or not; for as the needle in the perspective will always keep to the north of itself, if you do not perceive it has any motion, you conclude the crown is not in the box. It may happen, however, that the wire in the crown may be placed to the north, in which case you will be deceived. Therefore, to be sure of success, when you find the needle in the perspective remain stationary, you may make some pretence to desire the person to move the box into another position, by which you will certainly know if the crown-piece be there nor not.—You must remember, that the needle in the perspective must here be very sensible, as the wire in the crown cannot possibly have any great attractive force.

**2. The magnetic table.**

Under the top of a common table place a magnet that turns on a pivot; and fix a board under it, that nothing may appear. There may also be a drawer under the table, which you pull out to show that there is nothing concealed. At one end of the table there must be a pin that communicates with the magnet, and by which it may be placed in different positions: this pin must be so placed as not to be visible to the spectators. Strew some steel-filings or very small nails over that part of the table where the magnet is. Then ask any one to lend you a knife, or a key, which will then attract part of the nails or filings. Then placing your hand in a careless manner on the pin at the end of the table, you alter the position of the magnet; and giving the key to any person, you desire him to make the experiment, which he will then not be able to perform. You then give the key to another person; at the same time placing the magnet, by means of the pin, in the first position, when that person will immediately perform the experiment.

3. The mysterious watch.

You desire any person to lend you his watch, and ask him if he thinks it will or will not go when it is laid on the table. If he say it will, you place it over the end of the magnet, and it will presently stop (a). You then mark with chalk, or a pencil, the precise point where you placed the watch; and moving the position of the magnet, as in the last experiment, you give the watch to another person, and desire him to make the experiment; in which he not succeeding, you give it to a third person, at the same time replacing the magnet, and he will immediately perform the experiment.

4. The magnetic dial.

Provide a circle of wood or ivory, of about five or six inches diameter, as fig. 9, which must turn quite free on the land B (fig. 10.) in the circular border A: on the circle must be placed the dial of pasteboard C (fig. 9.), whose circumference is to be divided into 12 equal parts, in which must be inscribed the numbers from 1 to 12, as on a common dial. There must be a small groove in the circular frame D, to receive the pasteboard circle: and observe, that the dial must be made to turn so free, that it may go round without moving the circular border in which it is placed. Between the pasteboard circle and the bottom of the frame, place a small artificial magnet E, (fig. 11.) that has a hole in its middle, or a small protuberance. On the outside of the frame place a small pin P, which serves to show where the magnetic needle I, that is placed on a pivot at the centre of the dial, is to stop. This needle must turn quite free on its pivot, and its two sides should be in exact equilibrium. Then provide a small bag, that has five or six divisions, like a lady's work-bag, but smaller. In one of these divisions put small square pieces of pasteboard, on which are wrote the numbers from 1 to 12, and if you please you may put several of each number. In each of the other divisions you must put 12 or more like pieces; observing, that all the pieces in each division must be marked with the same number. Now the needle being placed upon its pivot, and turned quickly about, it will necessarily stop at that point were the north end of the magnetic bar is placed, and which you previously know by the situation of the small pin in the circular border. You therefore present to any person that division of the bag which contains the several pieces on which is wrote the number opposite to the north end of the bar, and tell him to draw any one of them he pleases. Then placing the needle on the pivot, you turn it quickly about, and it will necessarily stop, as we have already said, at that particular number.

Another experiment may be made with the same dial, by desiring two persons to draw each of them one number out of two different divisions of the bag; and if their numbers, when added together, exceed 12, the needle or index will stop at the number they exceed it; but if they do not amount to 12, the index will stop at the sum of those two numbers. In order to perform this experiment, you must place the pin against the number 5, if the two numbers to be drawn from the bag be 10 and 7; or against 9, if they be 7 and 2.—If this experiment be made immediately after the former, as it easily may, by dexterously moving the pin, it will appear the more extraordinary.

5. The dexterous painter.

Provide two small boxes, as M and N (fig. 12.), four inches wide, and four inches and a half long. Let the box M be half an inch deep, and N two-thirds of an inch. They must both open with hinges, and shut with a clasp. Have four small pieces of light wood, (fig. 13., 14., 15., 16.) of the same size with the inside of the box M (fig. 12.), and about one third of an inch thick. In each of these let there be a groove, as A, B, EF, CD, GH: these grooves must be in the middle, and parallel to two of the sides. In each of these grooves place a strong artificial magnet, as fig. 17. The poles of these magnets must be properly disposed with regard to the figures that are to be painted on the boards; as is expressed in the plate. Cover the bars with paper, to prevent their being seen; but take care, in putting it on, not to wet the bars, as they will thereby rust, which will considerably impair their virtue. When you have painted such subjects as you choose, you may cover them with a very thin clear glass. At the centre of the box N, place a pivot (fig. 18.), on which a small circle of pasteboard OPQR (fig. 19.) is to turn quite free; under which is to be a touched needle S. Divide this circle into four parts, which are to be disposed with regard to the poles of the needle, as is expressed in the figure. In these four divisions you are to paint the like subjects as are on the four boards, but reduced to a smaller compass. Cover the inside of the top of this box with a paper M, (see fig. 12.), in which must be an opening D, at about half an inch from the centre of the box, that you may perceive, successively, the four small pictures on the pasteboard circle just mentioned. This opening is to serve as the cloth on which the little painter is supposed to draw one of the pictures. You may cover the top of the box, if you please, with a thin glass. Then give the first box to any person, and tell him to place any one of the four pictures in it privately, and, when he has closed it, to give it you. You then place the other box over it; when the moveable circle, with the needle, will turn till it comes in the same position with the bar in the first box. It will then appear that the little dexterous painter has already copied the picture that is inclosed in the first box.

6. The cylindric oracle.

Provide a hollow cylinder of about six inches high and three wide, as A.B. Its cover CD must be made to fix on any way. On one side of this box or cylin-fig. 1. der, let there be a groove, nearly of the same length with that side; in which place a small steel bar (fig. 2.) that is strongly impregnated, with the north pole next the bottom of the cylinder. On the upper side of the cover describe a circle; and divide it into ten equal parts, in which are to be wrote the numbers from 1 to 10, as is expressed in fig. 3. Place a pivot at

(a) To perform this experiment, you must use a strong magnetic bar; and the balance of the watch must not be of base, but steel. the centre of this circle, and have ready a magnetic needle. You are then to provide a bag, in which there are several divisions, like that described in exper. 4. In each of these divisions put a number of papers, on which the same or similar questions are wrote. In the cylinder put several different answers to each question, and seal them up in the manner of small letters. On each of these letters or answers is to be wrote one of the numbers of the dial or circle at the top of the box. You are supposed to know the number of the answers to each question. You then offer one of the divisions of the bag, observing which division it is, to any person, and desire him to draw one of the papers. Next put the top on the cylinder, with that number which is wrote on the answer directly over the bar. Then placing the needle on the pivot, you turn it briskly about, and it will naturally flop at the number over the bar. You then desire the person who drew the question to observe the number at which the needle stands, and to search in the box for a paper with the same number, which he will find to contain the answer.—You may repeat the experiment by offering another division of the bag to the same or another person; and placing the number that corresponds to the answer over the magnetic bar, proceed as before.

It is easy to conceive of several answers to the same question. For example, suppose the question to be, Is it proper to marry?

Answer 1. While you are young not yet, when you are old not at all.

2. Marry in haste, and repent at leisure.

3. Yes, if you can get a good fortune; for something has some favour, but nothing has no flavour.

4. No, if you are apt to be out of humour with yourself; for then you will have two persons to quarrel with.

5. Yes, if you are sure to get a good husband (wife); for that is the greatest blessing of life. But take care you are sure.

6. No, if the person you would marry is an angel; unless you will be content to live with a devil.

7. The enchanted ewer.

Fix a common ewer, as A, (fig. 4.) of about 12 inches high, upon a square stand BC; in one side of which there must be a drawer D, of about four inches square and half an inch deep. In the ewer place a hollow tin cone, inverted, as AB, fig. 5. of about four inches and a half diameter at top, and two inches at bottom; and at the bottom of the ewer there must likewise be a hole of two inches diameter.

Upon the stand, at about an inch distance from the bottom of the ewer, and directly under the hole, place a small convex mirror H, of such convexity that a person's visage, when viewed in it, at about 15 inches distance, may not appear above two inches and a half long.

Upon the stand likewise, at the point I, fig. 2. place a pivot of half an inch high, on which must be fixed a touched needle RQ, inclosed in a circle of very thin pasteboard OS, fig. 6. of five inches diameter. Divide this pasteboard into four parts, in each of which draw a small circle: and in three of these circles paint a head as x, y, z, the drefs of each of which is to be different, one, for example, having a turban, another a hat, and the other a woman's cap. Let that part which contains the face in each picture be cut out, and let the fourth circle be entirely cut out; as it is expressed in the figure. You must observe, that the poles of the needle are to be disposed in the same manner as in the plate.

You are next to provide four small frames of wood or pasteboard, n° 1. 2. 3. 4. each of the same size with the inside of the drawer. On these frames must be painted the same figures as on the circular pasteboard; with this difference, that there must be no part of them cut out. Behind each of these pictures place a magnetic bar, in the same direction as is expressed in the plate; and cover them over with paper, that they may not be visible. Matters being thus prepared, you first place in the drawer the frame n° 4., on which there is nothing painted. You then pour a small quantity of water into the ewer, and desire the company to look into it, asking them if they see their own figures as they are. Then you take out the frame n° 4., and give the three others to any one, desiring him to choose in which of those drefs he would appear. Then put the frame with the dref he has chose in the drawer; and a moment after, the person looking into the ewer will see his own face surrounded with the drefs of that picture. For, the pasteboard circle (divided, as above described, into four parts, in three of which are painted the same figures as on three of the boards, and the fourth left blank) containing a magnetic needle, and the four boards having each a concealed magnet; therefore, when one of them is put in the drawer under the ewer, the circle will correspond to the position of that magnet, and consequently the person looking into the top of the ewer will see his own face surrounded with the head-drefs of the figure in the drawer.—This experiment, well performed, is highly agreeable. As the pasteboard circle can contain only three heads, you may have several such circles, but you must then have several other frames; and the ewer must be made to take off from the stand.

8. The box of metals.

Provide a wooden box, about thirteen inches long and seven wide, as ABCD (fig. 7.). The cover of this box should be as thin as possible. Have six small boxes or tablets, about an inch deep, all of the same size and form, as EFGHIK, that they may indiscriminately go into similar holes made in the bottom of the large box. In each of these tablets is to be placed a small magnetic ball, and their poles are to be disposed as expressed in the figure. Cover each of these tablets with a thin plate of one of the six following metals, viz. gold, silver, copper, iron, pewter, and lead. You must also have a magnetic perspective, at the end of which is to be two circles, one divided into six equal parts, and the other into four, as in fig. 8. from the centre of which there must be drawn an index N, whose point is to be placed to the north. Therefore, when you are on the side CD of the box, and hold your perspective over any one of the tablets that are placed on the holes E, F, G, so that the index drawn on the circle is perpendicular to the side AB, the needle in the perspective will have its south pole directed to the letter that denotes the metal contained in that that tablet. When you hold the perspective over one of the boxes placed in the holes H, I, K, so that the index drawn on the circle is perpendicular to the side CD, the fourth pole of the needle will in like manner express the name of the metal inclosed. If the under-side of any one of the tablets be turned upward, the needle will be slower in its motion, on account of the greater distance of the bar. The gold and silver will still have the same direction; but the four other metals will be expressed by the letters on the interior circle. If any one of the metals be taken away, the needle will not then take any of the above directions, but naturally point to the north; and its motion will be much slower. You therefore give the box to any one, and leave him at liberty to dispose all the tablets in what manner and with what side upward he pleases, and even to take any one of them away. Then, by the aid of your perspective, you tell him immediately the name of the metal on each tablet, and of that he has taken away.

This box of metals will, on comparison, be found far to exceed that which has been publicly exhibited: for that, being composed of six tablets, of which two only differ in form, admits but of six different dispositions, whereas in this the tablets may be placed 720 different ways. In the other, you must also know the particular side of the box, which in this is not at all necessary. Nay, you may here distinguish each metal, though the box be completely covered with paper; for the effect of the needle will be always the same. The experiments with this box are therefore much more extraordinary, and its construction at the same time more simple.

9. The magnetic planetarium.

Construct a round box, ILMN, (fig. 9.), of eight or nine inches diameter, and half an inch deep. On its bottom fix a circle of pasteboard, on which draw the central circle A, and the seven circumjacent circles B, C, D, E, F, G, H. Divide the central circle into seven equal parts by the lines AB, AC, AD, AE, AF, AG, and AH, which must pass through the centres of the other circles, and divide each of them into two equal parts. Then divide the circumference of each of those circles into 14 equal parts, as in the figure. You are likewise to have another pasteboard of the same figure, and divided in the same manner, which must turn freely in the box, by means of an axis placed on a pivot; one end of which is to be fixed in the centre of the circle A. See fig. 10. On each of the seven smaller circles at the bottom of the box, place a magnetic bar, two inches long, in the same direction with the diameters of those circles, and their poles in the situations expressed in the figure. There must be an index O, like that of the hour-hand of a dial, which is to be fixed on the axis of the central circle, and by which the pasteboard circle in the box may be turned about. There must be also a needle P, which must turn freely on the axis, without moving the circular pasteboard.—In each of the seven divisions of the central circle write a different question; and in another circle, divided into 12 parts, you may write the names of the 12 months. In each of the seven circles write two answers to each question, observing that there must be but seven words in each answer; in the following manner. In the first division of the circle G, which is opposite to the first question, write the first word of the first answer. In the second division of the next circle, write the second word; and so on to the last word, which will be in the seventh division of the seventh circle. In the eighth division of the first circle, write the first word of the second answer; in the ninth division of the second circle, write the second word of the same answer; and so on to the 14th division of the seventh circle, which must contain the last word of that answer. The same must be done for all the seven questions; and to each of them must be assigned two answers, the words of which are to be dispersed through the seven circles. At the center of each of these circles place a pivot; and have two magnetic needles, the pointed end of one of which must be north, and the other south, (QR). Now, the index of the central circle being directed to any one of the questions, if you place one of the two magnetic needles on each of the seven lesser circles, they will fix themselves according to the direction of the bars on the correspondent circles at the bottom of the box, and consequently point to the seven words that compose the answer. If you place one of the other needles on each circle, it will point to the words that are diametrically opposite to those of the first answer, the north pole being in the place of the south pole of the other.—You therefore present this planetarium to any person, and desire him to choose one of the questions there wrote; and you then set the index of the central circle to that question, and putting one of the needles on each of the seven circles, you turn it about; and when they all settle, they will point to the seven words that compose the answer. The two answers may be one favourable and the other unfavourable; and the different needles will serve to diversify the answers when you repeat the experiment.

There may be also a moveable needle to place against the names of the months; and when the party has fixed upon a question, you place that needle against the month in which he was born, which will give the business an air of more mystery. On the centre of the large circle may be the figure of the sun; and on each of the seven smaller circles one of the characters of the five planets, together with the earth and moon. This experiment, well executed, is one of the most entertaining that magnetism has produced.

10. The sagacious swan.

Providing a box XY, 18 inches long, nine wide, and two deep, the top of which is to slide on and off at the end Y. Toward the end X, describe a circle of fig. 1, six inches diameter, round which are to be fixed six small vases of wood or ivory, of one inch and a half high; and to each of them there must be a cover. At the end Y place an egg B, of ivory or other matter, of about three inches and a half high, with a cover that shuts by a hinge, and fastens with a spring. It must be fixed on the stand C; through which, as well as the bottom of the egg, and the part of the box directly underneath, there must pass a hole of one-third of an inch in diameter. In this cavity place an ivory cylinder F, that can move freely, and rises or falls by means of the spring R. You must have a thin copper basin A, of six inches diameter, which is to be placed on on the centre of the circle at X, and consequently in the middle of the six vases. Let a proper workman construct the movement expressed by fig. 2, which is composed of a quadrant G, that has 16 teeth, and is moveable about an axis in the stand H, that has an elbow, by which it is screwed to the bottom of the box at L. To the quadrant there must be joined the straight piece K. The horizontal wheel M has 24 teeth; and is supported by the piece S, which is screwed to the end of the box next Y. On the axis of this wheel place a brass rod O P, five inches long; and at the part O place a large bar or horse-shoe, of a semicircular form, and about two inches and a half diameter, strongly impregnated. The steel rod V, takes at one end the teeth of the quadrant G, by the pinion F, and at the other end the wheel M, by the perpendicular wheel N, of 30 teeth; the two ends of this rod are supported by the two stands that hold the other pieces. Under the piece K, that joins to the quadrant, must be placed the spring R, by which it is raised, and pushes up the cylinder that goes thro' the stand C into the egg. You must also have six small etwees or cases, as Y, fig. 3. They must be of the same circumference with the cylinder in the stand, and round at their extremities; their length must be different; that, when they are placed in the egg, and the lower end enters the hole in which is the cylinder, they may thrust it down more or less, when the top of the egg, against which they press, is fastened down; and thereby lower the bar that is fixed to the end of the quadrant, and consequently, by means of the pinion (fig. 4.) and wheels N M (fig. 2.), turn the horse-shoe that is placed upon the axis of the last wheel. The exact length of these etwees can be determined by trials only; which trials, however, may be made with round pieces of wood. In each of these etwees place a different question, wrote on a slip of paper and rolled up, and in each of the vases put the answer to one of the questions; as you will know, by trials, where the magnetic bar or horse-shoe will stop.

Lastly, provide a small figure of a swan, or what other you please, made of cork or enamel, in which you must fix a touched needle, of the largest size of those commonly used in sewing.

Being thus prepared, you offer a person the six etwees, and desire him to choose any one of them himself, and conceal the others, or give them to different persons. He is then to open his etwee, read the question it contains to himself, and return the etwee to you, after replacing the question. You then put the etwee in the egg, and, placing the swan upon the water in the basin, you tell the company the will presently discover in which of the vases the answer is contained. The same experiment may be repeated with all the etwees.

11. The multiformis verse.

The eight words that compose this Latin verse,

Tot sunt tibi dotes, quot celi sidera, virgo (A),

being privately placed in any one of the different combinations of which they are susceptible, and which are 40320 in number, to tell the order in which they are placed.

Provide a box that shuts with hinges, and is eight inches long, three wide, and half an inch deep. Have eight pieces of wood about one-third of an inch thick, two inches long, and one and a half wide, which will therefore, when placed close together, exactly fill the box. In each of these pieces or tablets place a magnetic bar, with their poles as is expressed in the figure. The bars being covered over, write on each of the tablets, in the order they then stand, one of the words of the foregoing Latin verse. On a very thin board of the same dimensions with the box, draw the eight circles, A, B, C, D, E, F, G, H, (fig. 12.) whose centres should be exactly over those of the eight tablets in the box when the board is placed upon it. Divide each of those circles into eight parts, as in the figure; and in each of those divisions write one of the words of the Latin verse, and in the precise order expressed in the plate; so that, when the board is placed over the box, the eight touched needles placed at the centre of the circles may be regulated by the poles of the bars in the box, and consequently the word that the needle points to in the circle be the same with that inscribed on the tablet. Cover the board with a glass, to prevent the needles from rising off their pivots, as is done in the compass. Over the board place four plates of glass, I, L, M, N, fig. 13. which will give the machine the figure of a truncated pyramid, of eight inches high. Cover it with a glass, or rather a board in which are placed two lenses, O O, of eight inches focus, and distant from each other about half an inch. Line the four plates of glass that compose the sides with very thin paper, that will admit the light, and at the same time prevent the company from seeing the circles on the board.

These preparations being made, you give the box to any one; and tell him to place the tablets on which the words are wrote, privately, in what position he thinks proper, then to close the box, and, if he please, to wrap it up in paper, seal it, and give it you. Then placing the board with the pyramid upon it, you immediately tell him the order in which the tablets are placed, by reading the words to which the needles on the circles point.

MAGNITUDE, whatever is made up of parts locally extended, or that hath several dimensions; as a line, surface, solid, &c.