for, heated red-hot and cooled in the meridian from north to south, they presently contract a polary power, and being poised in air or water, convert that part unto the north which respected that point in its refrigeration; so that if they had no sensible verticity before, it may be acquired by this way, or if they had any, it might be exchanged by contrary position in the cooling. For by the fire they amit not only many drossy and scorious parts, but whatsoever they had received either from the earth or loadstone; and so being naked and despoiled of all verticity, the magnetical atoms invade their bodies with more effect and agility.6

turbed; for after some days it was found that the power of the bar continued just the same as at the time of making the experiment when it had not been displaced; but then the bar upon trial was always found to possess a certain degree of fixed magnetism, its other extremity producing an opposite effect upon the needle; but if the bar was inverted, while it retained any visible colour from the heat, both ends produced exactly the same deflection: as to the magnetic effect to which we have alluded above, it was lost, or at least a great part of it, after leaving the bar for some time horizontal, or, after its being thrown about with other pieces of iron.”—Br.

whatsoever they had

For by the fire they amit, &c. received either from the earth or loadstone; and so, &c.] This statement is true in itself, but unless viewed in connexion with other facts it may produce an erroneous impression upon the mind. Mr. Barlow's experiments have also elucidated this subject; his results on which will be appropriately introduced by an historical notice of it, derived from the Treatise on Magnetism cited in the preceding note, from which the comparative amount and nature of our author's knowledge respecting it may be inferred, by contrasting it with his own remarks. "The effect of temperature, in changing and destroying the magnetic power of iron bodies had been long imperfectly known, but it had never been satisfactorily established prior to Mr. Barlow's experiments. It is, for example, stated in Newton's Optics, that red-hot iron has no magnetic property, while Father Kircher asserts, that the magnet will attract red-hot iron as well as cold; Mr. Cavallo again found, that although iron at a red heat had a greater power over the magnet than when cold, yet at the white heat it had less; but he was not aware that it was entirely lost at a white heat."—(Encyc. Metrop. Magnetism, § vi. 41.) The last-mentioned fact, viz. that the magnetic action of iron is destroyed by a white heat, was ascertained by Mr. Barlow in the experiments already noticed, and he observed, on the same occasion, an extraordinary phænomenon, the consideration of which will explain some of the apparently contradictory facts which are related by our author in this chapter. It is, that "after the iron loses its entire power of attraction at the white heat, it acquires, as that colour subsides into the bright red and red, an attractive power, the reverse of what it had

Neither is it only true what Gilbertus first observed, that irons refrigerated north and south acquire a directive faculty, but, if they be cooled upright and perpendicularly, they will also obtain the same: that part which is cooled towards the north, on this side of the equator, converting itself unto the north, and attracting the south point of the needle; the other and highest extreme respecting the south, and attracting the northern, according unto the laws magnetical: for (what must be observed) contrary poles or faces attract each other, as the north the south; and the like decline each other, as the north the north. Now on this sides of the equator, that extreme which is next the earth is animated unto the north, and the contrary unto the south, so that in coition it applies itself quite oppositely, the coition or attraction being contrary to the verticity or direction. Contrary, if we speak according unto common use, yet alike, if we conceive the virtue of the north pole to diffuse itself, and open at the south, and the south at the north again.

This polarity from refrigeration, upon extremity, and in defect of a loadstone, might serve to invigorate and touch a needle any where; and this, allowing variation, is also the readiest way at any season to discover the north or south; and surely far more certain than what is affirmed of the been when cold; so that if the bar and needle are so situated that the north end [of the needle] is attracted when the iron is cold, the south end will be attracted during the above interval," that is, while it is passing through the high temperatures indicated by the shades of colour just mentioned; after which the usual attractive power again takes place in the increased degree stated in the preceding note.-Br.

7 but if they be cooled upright, &c.] This statement is quite accurate ; and, in fact, a nearly perpendicular position is more favourable to the reception of magnetism by the iron than the horizontal. The effect depends upon the suitable position of the iron for receiving and retaining a portion of the magnetism of the earth, and the position in which the greatest effect is exerted by the earth's magnetism is when the iron is placed in the position of the dipping needle; that is, inclined to the magnetic north at an angle (in these latitudes, and at the present time) of about 69 degrees with the horizon. The subject is resumed, and with equal correctness, towards the end of the next paragraph, which has evidently been written from experiment. Both are replete with just representations of the facts.-Br.

Now on this side, &c.] Itt is certainly knowne that beyond the line the needle keeps his posture to the north, as truly as att the first setting out of England.—Wr.

up

grains and circles in trees, or the figure in the root of fern. For if we erect a red-hot wire until it cool, then hang it with wax and untwisted silk, where the lower end and that which cooled next the earth doth rest, that is the northern point; and this we affirm will still be true, whether it be cooled in the air or extinguished in water, oil of vitriol, aqua fortis, or quicksilver. And this is also evidenced in culinary utensils, and irons that often feel the force of fire, as tongs, fire-shovels, prongs, and andirons; all of which acquire a magnetical and polary condition, and, being suspended, convert their lower extremes unto the north; with the same attracting the southern point of the needle. For easier experiment, if we place a needle touched at the foot of tongs or andirons, it will obvert or turn aside its lily or north point, and conform its cuspis or south extreme unto the andiron. The like verticity, though more obscurely, is also contracted by bricks and tiles, as we have made trial in some taken out of the backs of chimneys. Now, to contract this direction, there needs not a total ignition, nor is it necessary the irons should be red-hot all over. For if a wire be heated only at one end, according as that end is cooled upward or downward, it respectively acquires a verticity, as we have declared in wires totally candent. Nor is it absolutely requisite they should be cooled perpendicularly, or strictly lie in the meridian; for, whether they be refrigerated inclinatorily or somewhat equinoxially, that is, towards the eastern or western points, though in a lesser degree, they discover some verticity.

Nor is this only true in irons, but in the loadstone itself.

9 The like verticity, &c.] The cause of this is doubtless the magnetism acquired by the particles of oxide of iron disseminated throughout the clay of which bricks and tiles are made, and which, of course, remain distributed in the same sensibly uniform manner in the bricks and tiles themselves. Each particle itself acquiring, by being placed in a position not greatly deviating from that of the dipping needle, magnetical polarity of the same kind as the rest, the result is a general polarity of all, which, freely permeating the earthy matter, appears to be possessed by the brick or tile itself. Assuming the author's experiment to be correct (and this there is no reason to doubt), such must be the explanation of the facts. The subject will be resumed under another form, when commenting upon the passage "Of rocks magnetical," in the following chapter.-Br.

For if a loadstone be made red-hot, it loseth the magnetical vigour it had before in itself, and acquires another from the earth in its refrigeration; for that part which cooleth toward the earth will acquire the respect of the north, and attract the southern point or cuspis of the needle. The experiment hereof we made in a loadstone of a parallelogram or long square figure; wherein only inverting the extremes, as it came out of the fire, we altered the poles or faces thereof at pleasure.

It is also true what is delivered of the direction and coition of irons, that they contract a verticity by long and continued position; that is, not only being placed from north to south, and lying in the meridian, but respecting the zenith and perpendicular unto the centre of the earth; as is manifest in bars of windows, casements, hinges, and the like. For if we present the needle unto their lower extremes, it wheels about and turns its southern point unto them. The same condition in long time do bricks contract which are placed in walls, and, therefore, it may be a fallible way to find out the meridian by placing the needle on a wall; for some bricks therein, by a long and continued position, are often magnetically enabled to distract the polarity of the needle. And, therefore, those irons which are said to have been converted into loadstones, whether they were real conversions or only attractive augmentations, might be much promoted by this position: as the iron cross of an hundred weight upon the church of St. John, in Ariminum, or that loadstoned iron of Cæsar Moderatus, set down by Aldrovandus.*1

Lastly, irons do manifest a verticity, not only upon refrigeration and constant situation, but (what is wonderful, and advanceth the magnetical hypothesis) they evidence the same by mere position, according as they are inverted, and their extremes disposed respectively unto the earth. For if an iron or steel, not firmly excited, be held perpendicularly or inclinatorily unto the needle, the lower end thereof will attract the cuspis or southern point; but if the same extreme be inverted and held under the needle, it will then

*De Mineralibus.

And, therefore, those irons, &c.] Added in the 2nd edition.

attract the lilly or northern point; for by inversion it changeth its direction acquired before, and receiveth a new and southern polarity from the earth, as being the upper extreme. Now, if an iron be touched before, it varieth not in this manner; for then it admits not this magnetical impression, as being already informed by the loadstone, and polarily determined by its preaction.

And from these grounds may we best determine why the northern pole of the loadstone attracted a greater weight than the southern on this side the equator; why the stone is best preserved in a natural and polary situation; and why, as Gilbertus observeth, it respecteth that pole, out of the earth, which it regarded in its mineral bed and subterraneous position.

It is likewise true and wonderful, what is delivered of the inclination or declination of the loadstone; that is, the de

2 For if an iron or steel, &c.] The magnetism of the earth appears to emanate from it in curves, originating far within the earth, or perhaps at its centre, their planes being magnetic meridians, but which, for small distances, are sensibly straight lines. The angles which they form with the horizon, or, what is practically the same, with the earth's surface, is the angle of the dip of the needle for each latitude respectively, being, as already stated, about 69 degrees for the British Islands at the present time. The dipping needle, therefore, is nothing but a magnetized body freely obeying the tendency imparted to it by these curves, and, in fact, exhibiting their direction at each particular spot. Now, if a bar of soft iron, or other body susceptible of magnetism, but perfectly free from it, be held in the direction of the dipping needle it acquires polarity, for the time, the lower end becoming a south pole, attracting the north end of a compass needle; and the upper a north pole, attracting the south end of the needle. If the bar be inverted, but its direction still preserved, the end which was before the north pole will become the south pole, and vice versa. If, however, it has already received permanent magnetism, these effects do not take place; agreeably to our author's statement.—Br.

3 inclination or declination of the loadstone.] The phænomena described in this and the succeeding paragraph, are those of the dip of the magnetic needle, now usually observed by means of a needle placed in a circle divided into degrees, &c., in order to show the amount of the dip, or the angle formed with the horizon by the needle when allowed freely to obey the influence of terrestrial magnetism. The reader will be enabled to form correct ideas on this subject from the present brief remarks, if taken in conjunction with the three preceding notes, and also with the next, by the same annotator, in which the history of the dipping needle is continued.-Br.