agates by artificial means. It would be necessary to re-examine a collection of these substances, and to repeat some of the experiments on them, to determine the extent to which this art may be carried, and the exact nature of the varieties which are susceptible of the changes in question. As the discovery of the internal structure of agates is your own, no one is more competent than yourself to make these further inquiries, from which I am at this moment precluded. It has long been known, that zoned agates, formed of laminæ alternately black and white, were brought from India; but it is only since the peace that the same substances have been imported from Germany in considerable quantity; in consequence of which their value has fallen to little more than the price of cutting. These latter are coloured by an artificial process, which is a kind of secret in the trade, and it is not improbable that the specimens from India are produced in the same manner, as the natives of that country possess the art of staining the same minerals white. As the lapidaries are not acquainted with the theory of their process, they are very subject to failures, which also arise at times from the nature of the stones being unsusceptible of the colouring process. The common process consists in boiling the cut specimens in sulphuric acid; in consequence of which, a particular lamina, or set of laminæ, is rendered black, while others retain their natural colour, or even become whiter than before, thus producing that powerful contrast which is esteemed valuable in this stone. This process often fails, and will always fail, if tried on specimens which have not previously been cut on the lapidaries' wheel. It is, in fact, produced by the action of the sulphuric acid on the oil which has been absorbed by the stone in cutting, and can therefore very obviously be insured, by previously boiling in oil the specimens which are to be subjected to the blackening process. That this is the fact is proved, if proof were necessary, by the disengagement of sulphurous acid gas, which takes place during the action of the acid. To insure success, therefore, it is evident that either the application of the oil must be continued for a sufficient length of time, or that the stone be cut so thin as to admit of its being penetrated by it before the sulphuric acid is applied. You will easily see, that this ab sorbent property of agates explains the fact of their being occasionally blackened by sulphuric acid, so as to have led chemists to imagine that they naturally contained some carbonaceous matter; and it was a knowledge of this circumstance which led me to subject to long boiling in a solution of potash, those specimens in which I suspected vegetable remains to be entangled, before applying to them the test of sulphuric acid. The fact itself is curious in another point of view, as it indicates the porosity of agates, and is nearly connected with your interesting discoveries respecting their internal structure. In examining the specimens which are to be subjected to this treatment, it is in the first place evident, that the future changes are not indicated by the colours, as the red, or otherwise coloured laminæ, sometimes become black, while in others the opaque white, or the transparent, are affected. In the few specimens which I possessed at the time I was engaged on this subject, I had no means of determining in what respect the change was connected with apparent differences in the internal structure, and I can only now suggest it to you as a subject for examination. In most zoned agates, some of the lamina will be found exempt from any apparent internal structure, while in others the appearance of undulating fibres is evident, the fibres themselves varying materially in size. If any conjecture were to be formed a priori, it would be expected that the most distinctly fibrous laminæ were the most porous; but it is unnecessary to offer conjectures on what you may so easily put to the test of experiment. Having mentioned the Indian practice of colouring agates white, it may be remarked, that this is also a secret in the hands of lapidaries, although apparently not generally known to them, and but little practised. Beads of carnelian are sometimes brought from India, ornamented with reticulations of a white colour, penetrating to a small depth within the stone, and equally hard. The black agates of this kind, which are sometimes coloured with complicated or fine lines of white, are often very singular, and, without a knowledge of the mode in which they are produced, have a very puzzling appearance. They are thus coloured by applying carbonate of soda, and exposing them to the heat of a furnace or a muffle. An opaque white enamel is thus produced, which appears as refractory to steel as the original stone, and cannot easily be distinguished from a natural lamina of white, when used, as it has sometimes been, for producing flat specimens for cameos. By either of these modes, indeed, stones for engravers' work are easily formed, but in the method of blackening the susceptible lamina by sulphuric acid and oil, the effect is more brilliant, and the contrast of the black and white more decided. BANFF, July 1819. ART. XX.-Account of some important discoveries in Magnetism, recently made by P. BARLOW, Esq. one of the Professors of Mathematics in the Royal Military Academy, Woolwich *. THE Treatise on the Variation of the Compass, lately published by Mr Bain, and the magnetical observations made by Captain Ross and Captain Sabine, in the Arctic Regions, have turned the attention of men of science to the deviation produced by the action of the ship upon the needle of the compass. That eminent mathematician Dr Thomas Young, has constructed a formula and a table from the experiments made on board the Isabella, by which an approximate measure of the deviation may be obtained. Lieutenant Robertson of the Isabella, has also deduced general rules for the same purpose, and Mr Barlow, in investigating the subject experimentally, has been led to several interesting and important results, which could not have been anticipated from the known laws of the distribution of magnetism. At the commencement of this inquiry, his intention was to avail himself of the favourable opportunity furnished by the immense masses of iron contained in the Royal Arsenal at Woolwich, to make some experiments, with a view of sub • Through the kindness of one of our correspondents, who has seen Mr Barlow's experiments, and from other sources of information, we are enabled to present our readers with this early notice of them. Mr Barlow's paper was read at the Royal Society on the 20th May 1818, and will probably appear in the next part of the Transactions of that distinguished body.-ED. mitting to calculation the local effect of a ship's guns on the compass. He began his observations, on the effect of balls of different dimensions, and in the course of these, he was led to the following curious experimental fact, viz. that there is round every globe and mass of iron a great circle inclining from north to south, and forming with the horizon an angle of about 19 or 20 degrees, in the plane of which the iron has no effect in changing the direction of the needle, that is to say, while the centre of the needle is found in the plane of this circle, the compass will point north and south, the same as if no iron were present. The dip of the needle being about 70° 30', he apprehended that the inclination of this circle was equal to the complement of the dip; and subsequent experiments on an excellent horizontal compass and dipping needle, furnished by Mr Berge, have confirmed the accuracy of these surmises. This fact being established, his next object was to ascertain what law was observed in the attraction of iron when the compass was removed out of the above circle of no attraction; and with this view, he contrived, by means of an apparatus constructed for the purpose, to carry the compass round the ball (which was 13 inches in diameter, and solid, weighing 288 lb.) in various circles; and by these means succeeded in deducing a law of action, which was singularly uniform, the computed and observed results scarcely ever deviating from each other, by a quantity greater than the daily variation, viz. from 10 to 20, or at most 30 minutes. The nature of the above laws will be best conceived by the following artificial consideration. Call the circle of no attraction above described, the magnetic equator of a sphere circumscribing the ball, and its two poles, the poles of that sphere. Conceive now circles of latitude and longitude to be drawn, the first meridian of the latter passing through the east and west points of the horizon, and the magnetic equator. Then the law in question is this, That the tangent of the deviation of the needle from the north or south, is proportional to the rectangle of the sine of the double latitude and cosine of the longitude; so that, knowing the deviation in one instance only, it may be computed for any other whatever. It is obvious, that although these laws have been pointed out with reference to the circles supposed to be drawn on the iron ball, or circumscribed sphere, they actually appertain to the needle itself; for when the compass is situated in any particular latitude and longitude with respect to the circles of the ball, the centre of the latter is similarly situated with reference to a sphere conceived to circumscribe the compass, having its centre coinciding with the centre of the needle; and hence the rules become immediately applicable to the determination of the local effect of a ship's guns, viz. find by experiment or otherwise, the centre of attraction of all the ship's iron; then in any part of the world where the dip is known, and at any direction of the ship's head, find the latitude and longitude of the centre of attraction with reference to a sphere circumscribing the compass, as supposed above; and the effect of the attraction may then be computed by means of the above rules, the deviation in any one instance having been previously determined. The only thing that can be considered as doubtful in the above rule is this: Does the circle of no attraction every where incline to the horizon at an angle equal to the complement of the dip? Mr Barlow has little doubt that it does, but, to be assured of the fact, he conceives that experiments must be repeated in different parts of the world. Having ascertained the law of deviation as it regarded position, and which he found to hold in the most irregular masses of iron, Mr Barlow next proceeded to ascertain the law as it regards distance, and he found by the most unexceptionable results, that, all things else being the same, the tangents of the angles of deviation are reciprocally proportional to the cubes of the distances. And again, that, cæteris paribus, the tangents of deviation are directly proportional to the cubes of the diameters of the iron balls or shells, by which the deviation is produced. By combining these laws with those given above, Mr Barlow has found, that the whole may be expressed by the formula, Sin 2x. cos / D3 where A is the angle of deviation, a tan A= the latitude, A. d3 the longitude, D the diameter of the ball or shell, d the distance of the centre of attraction from the pivot |