MAGNETISM

M A G N E T I S M,

THE quality or constitution of a body, and its pores, whereby it is rendered magnetical, 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. Gaffendus indeed, as well as Merfennus 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 4584$\backslash frac\{1\}\{2\}$ grains lost 2$\backslash frac\{1\}\{2\}$ grains by the touch; and another of 65726 grains weight, no less than 14 grains.

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 af-

terwards 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 soever (provided it be good) they were excited by. Nor does the touch seem to do much more in magnetical, 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 part; at four times the distance, the power 32 times as small; and at six 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 assuring us, that the

Laws of Magnetism. 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 Hauksbee, 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 arcs of declination, as the true measures of the power of magnetism.

Distances in inches.	Degrees of inclination.	Sines of $\backslash frac\{1\}\{2\}$ arcs.	Rat. sesqui-dupl.
20	2	175	466
14 $\backslash frac\{1\}\{2\}$	4	349	216
13 $\backslash frac\{1\}\{2\}$	6	523	170
12 $\backslash frac\{1\}\{2\}$	8	697	138
11 $\backslash frac\{1\}\{2\}$	10	871	105
10 $\backslash frac\{1\}\{2\}$	12	1045	87
9 $\backslash frac\{1\}\{2\}$	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" $\backslash frac\{1\}\{2\}$, 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 Hellsham, trying the experiment by his loadstone, found it to be as the squares of the distances inversely; and Mr Martin assures 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 $\backslash frac\{1\}\{4\}$, $\backslash frac\{1\}\{8\}$, $\backslash frac\{1\}\{16\}$, 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.
Distances.	$\backslash frac\{1\}\{4\}$—156	156	56	156
	$\backslash frac\{1\}\{8\}$—50	39	19	56
	$\backslash frac\{1\}\{16\}$—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

Entertain-
ing
Experi-
ments.

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 is become 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 strew 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 AB and CD, 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 pasteboard, 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 glass, 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 or 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 stand 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 where 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 AB, 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 passing it on, not to wet the bars, as they will thereby ruin, 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 AB. Its cover CD must be made Plate CLXIV. to fix on any way. On one side of this box or cylinder 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 the

(A) To perform this experiment, you must use a strong magnetic bar; and the balance of the watch must not be of brass, but steel.

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

No. 1.

No. 2.

No. 3.

Fig. 11.

Fig. 12.

Fig. 7.

Fig. 8.

Fig. 9.

Fig. 10.

Fig. 13.

This image shows a single, blank page of aged paper. The paper has a light beige or cream color with a slightly textured appearance. There are several faint, irregular smudges and discolorations scattered across the surface, most notably a larger, faint smudge in the upper right quadrant and some darker, more concentrated staining along the bottom right edge. The overall appearance is that of an old, unused page, possibly an endpaper or flyleaf from an antique book.

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 stop 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 favour.

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 dress of each of which is to be dif-

ferent, 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^o 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^o 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^o 4. and give the three others to any one, desiring him to choose in which of those dresses he would appear. Then put the frame with the dress he has chose in the drawer; and a moment after, the person looking into the ewer will see his own face surrounded with the dress 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-dress 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 south 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 please, 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 an-

swer; 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.

PROVIDE a box XY, 18 inches long, nine wide, and two deep, the top of which is to slide on and off at plate CLXV. 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

Entertain-
ing
Experi-
ments.

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 OP, five inches long; and at the part O place a large bar or horse-shoe, of a femicircular 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 con-

tained. The same experiment may be repeated with all the etwees.

11. The multifarious verse.

THE eight words that compose this Latin verse,
Tot sunt tibi doles, quot exili 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 sea-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.

M A G

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

MAGNIFYING, the making of objects appear larger than they would otherwise do; whence convex lenses, which have the power of doing this, are called magnifying glasses. See OPTICS.

MAGNOLIA, the LAUREL-LEAVED TULIP-TREE; a genus of the polygynia order, belonging to the po-
VOL. VI.

M A G

yandria class of plants.

Species. 1. The glauca or small magnolia is a native of Virginia, Carolina, and other parts of North America. In moist places it rises from seven or eight to 15 or 16 feet high, with a slender stem. The wood is white and spongy, the bark smooth and of a greenish white colour; the branches garnished with thick smooth leaves, like those of the bay; but of an oval shape, smooth on their edges, and white underneath.

(A) i.e. Thy virtues, virgin, are as numerous as the stars of heaven.