[The paper closes with an enumeration of the measurements of crystals of the Achmatowsk Clinochlore with a Mitscherlich's goniomer, by which the angles were obtained. The mean for P: M from 12 measurements of one crystal was 113° 58'; (the extremes 113° 57′ and 113° 58′) from another 113° 56'; from a third 113° 56'; giving for the mean of the three 113° 56' For M M from No. 1, 125° 38', for No. 2, 125° 37'. For P: 0 from crystal No. 3, 102° 64'; from crystal No. 5, 1020 6'; from crystal No. 3, P: n=118° 28'; from No. 6, P : t=108° 11′; from No. 2, M: t=124° 3; from Nos. 2 and 3, t: n=124° 32'; from No. 4, 124° 30'; from No. 4, P: x=125° 4. From No. 7, P: d=119° 5′, m: i=150° 0'.]

ART. XVIII.-A brief notice of some facts connected with the Duck Town, Tennessee, Copper Mines; by M. TUOMEY, State Geologist, Ala.

KNOWING that Dr. Curry of Nashville, and Mr. Proctor of the Tennessee Mines, are preparing a memoir, that will include the history, geological and economical relations of these mines, I intend to confine myself to a fact or two that I observed during a hurried visit to the place, and which may be worth being placed upon record.

Notwithstanding the excitement produced by the discovery of these mines, the current accounts of the richness of the ore, and great thickness of the beds, are not greatly exaggerated.

In a short but very lucid report, Mr. Whitney has presented a clear view of the state of some of these mines which he was commissioned to examine, together with a section which represents the geological character of the country for a distance of 40 miles. The road to the mines, which lies along the banks of the Ocoee river, exposed an admirable section of 24 miles in length. The mines are situated at the

junction of the silurian and metamorphic rocks, or as Mr. Whitney suggests, the cupriferous slates may be altered silurian strata.

The accompanying cut presents the position, and relation of the metalliferous beds at all the mines: a, upper portion of the bed composed of a porous, amorphous mass of red and brown oxyd of iron, the gossan of the Cornish miners, iron hat of the Germans, which is the residue of the ore after the copper

has been dissolved out. This process, the solution of the ore, is constantly, though, of course, slowly going on, as may be seen at the works, and hence the very variable thickness of the ore b. From portions of the bed every trace of copper has disappeared, down to c. The copper ore b, is a bluish black altered sulphuret. That the alteration is due to heat is rendered highly probable by the vesicular structure of the ore itself, as well as by the joints with which it is intersected, and which correspond in direction with those of the surrounding rocks of the country.

In every published account of these mines that I have seen, the impression is left, that the ore b is derived from the underlying portion of the bed c, by decomposition. Now c has been reached in all the mines, and invariably consists of arsenical iron, with rarely more than one per cent. of copper in the form of yellow sulphuret, and consequently could not furnish by decomposition or any other conceivable process, an ore containing 20 per cent. of that metal.

This lower arsenical iron portion c, of the bed is found everywhere immediately underlying the black ore, at no great distance below the surface, and is frequently met with even in the levels driven in the hill-side.

The whole of that portion of the bed above c, doubtless once consisted of yellow sulphuret of copper, and the part below c, as at present, of arsenical iron. During the metamorphism of the slates the sulphuret was altered to the black ore, and subsequently the soluble salts derived from this ore, were dissolved out by the simple process of leaching, the residual gossan, or iron hat being left in the upper portion of the bed, and the still unbleached ore, resting on the arsenical iron.

It is remarkable that this same arsenical iron with a little copper, is found in some of the shafts sunk, in exploring for copper in Alabama.

The solution of the question, What is below the arsenical iron? is a most interesting subject in connection with the value and future prospects of the mines. At one place a shaft has been sunk in the arsenical iron, to a depth of 10 or 12 fathoms without showing any encouraging change, and at another a shaft has been commenced calculated to cut the bed, 50 fathoms below the present level of the mine, or, as it is called in the mining reports, for the purpose of "proving the yellow sulphuret." These experimental shafts, if continued, will be of the utmost importance to the whole copper region, including portions of Georgia and Alabama. Should this arsenical iron terminate, at a moderate depth, in the yellow sulphuret, then indeed may Tennessee boast of such mines as are not found in the history of mining operations.

ART. XIX.-On the Periodical Variations of the Declination and Directive Force of the Magnetic Needle; by W. A. Norton, Professor of Civil Engineering in Yale College.

THE direction in which the magnetic force of the earth solicits either end of a magnetic needle is ascertained by suspending the needle freely by its centre of gravity. It is well known that throughout the greater part of the northern hemisphere the north end of a needle thus suspended is depressed below the plane of the horizon; and that throughout the greater part of the southern hemisphere the south end is depressed below this plane; also that this inclination or dip of the needle gradually increases from the magnetic equator, where it is zero, in both directions to the magnetic poles, where it is 90°. Such then are the varying directions of the directive force of the magnetic needle at different points of the earth's surface. The two ends or poles of the needle are solicited in opposite directions; the north end downward, and the south end upward. It suffices in discussing the perturbations to which the earth's magnetic force is subject, whether of direction of intensity, to confine our attention to the action upon one end of the needle, for example the depressed end,-(north end in the northern hemisphere, and south in the southern hemisphere); since the repelling force, acting upon the more elevated end, experiences corresponding changes, and has uniformly the same tendency, in giving direction to the needle. If now the directive force be decomposed into two components, the one horizontal, and the other vertical, we have what are called the horizontal force, and the vertical force, acting upon the needle; or the horizontal and vertical magnetic intensities of the place. These two forces, together with the declination (or deviation of the line of direction taken up by the horizontal compass needle, from the true north and south line) constitute what are called the Magnetic Elements of the station. Each of these elements changes in value as we pass from one station to another. It is well known also that at any one station they are not at all times unalterably the same; but are subject to variation from hour to hour, from day to day, and from year to year. The changes from hour to hour during the period of a day we call the Diurnal Variations; those from day to day, during the period of a year, we call the Annual Variations; and the change from year to year, the Secular Change. In a more restricted sense we call the entire amount of the change, whether of increase or diminution, in a day, the diurnal variation of the element; and we often attach a similar signification to the term annual variation, of a magnetic element. But, in the present paper I shall generally use these terms in the most

comprehensive sense; that is as comprising all the variations that occur during the day, whether we compare them hour by hour, or by longer intervals.

There are two classes of diurnal variations of the magnetic elements-viz, those which are Periodic and those which are Irregular, or more properly Occasional. Thus any one element, as the declination, regularly increases during a certain portion of every day, and then as regularly decreases during another portion of the day. Changes also occur, to all appearance fortuitously, at any hour of the day; so that their occurrence cannot be predicted for any one hour. Still it is now known that the irregular variations, so called, are under the control of certain overshadowing laws; thus they occur more frequently, and are larger in amount at certain hours than at other hours, and at certain parts of the day the liability is to an increase, at other parts to a diminution. If we compare the amount of the declination, or other element, at any hour of the day, with the same at the same hour of the following day, we find that a change has occurred, and if we do the same throughout the year we discover that the element in question, or its amount at a particular hour increases during half of the year, from a certain day, and decreases during the remaining half; in other words, it undergoes a regular variation, the period of which is a year. The amount of the change of the declination, or horizontal or vertical force, that takes place during a day, also varies from one season to another. Besides the periodic variations whose period is a day, or half a day, or a year, there is another class, recently discovered, whose period is about ten years (10 to 11 years). Thus, it is found that the amount of the alternate increase and decrease of the declination during a day is greater some years than others, and that it alternately augments and diminishes during a period of ten or eleven years. It is an interesting and very important fact, in a physical point of view, that this period has been found to be identical with that of a change which has been observed to occur in the number and magnitude of the spots on the sun from year to year, the maximum and minimum of the one quantity coinciding also in point of time with the maximum and minimum of the other quantity. Thus the year 1843 was that in which the mean daily movement of the needle, for the year, was the least, and also that in which the number and magnitude of the solar spots was the least. The year 1838 was the epoch of the previous maximum for both. The changes are very marked; the number of groups of solar spots observed by M. Schwabe during the year 1843 was 34, during the year 1838, 282; the mean daily movement of the needle at Toronto for the entire year, was 890 in 1843, and 12/11 in 1848, (the year of the following maximum). It has been still more recently established,

that the irregular variations, so called, undergo similar changes during the period just mentioned-the solar period, as it may be termed, by way of distinction.

It is now about twenty-five years since the project was conceived, by Baron Humboldt, and partially carried into execution, of covering the earth with magnetical observatories, at which "simultaneous observations should be made of every regular and irregular excitement of the earth-force." Since the year 1840, magnetical observatories have been in systematic operation in all parts of the earth, "from Toronto, in Upper Canada, to the Cape of Good Hope, and Van Diemen's Land, from Paris to Pekin;" provided with the magnetometers contrived by Gauss in 1832. These are large magnets delicately suspended, and carrying a small mirror, in which the observer, looking through a small telescope firmly fixed on a stone pier, sees the reflection of the fixed scale, and thus observes with great precision the smallest movement of the magnet. The horizontal force magnetometer is brought by the torsion of its suspension wire into a position at right angles to the magnetic meridian, and shows by its motions the variations of this force. The vertical force magnetometer devised by Dr. Loyd of Dublin, admits of motion only in a plane perpendicular to the magnetic meridian. The bar rests by a knife edge, on agate planes, and is adjusted by a ball moveable upon a fine screw, so as to deviate a little from the truly vertical position. In its improved form it carries a mirror at right angles to the plane of the bar, like the declination and horizontal force magnetometers. With these magnetometers a change of magnetic intensity amounting to the 5th part is measured. changes of declination are shown by the declination magnetometer, or declinometer, to within 2" to 3" of the truth. In this "net work of stations provided with similar instruments" are especially to be noticed the British Colonial Observatories (at Toronto, St. Helena, Cape of Good Hope, and Hobarton in Van Diemen's Land) which began operations in 1840, and the Russian Observatories scattered over the Russian Empire, which were erected about ten years earlier. Hourly or bi-hourly observations have been made at these and at many other observatories in Europe and elsewhere, for many years; and on certain days, called Term Days, they have been noted as often as every 2 minutes. We have now several volumes of Reports of the magnetical and meteorological observations made at the British Colonial Observatories, with abstracts, discussions, &c., published under the direction of Colonel Edward Sabine. Annual Reports of the observations made at the Russian Observatories have also been published, under the superintendence of Professor Kupffer. Reports of the observations made at the Girard College Observatory, and also at Washington, under the direction respectively of Professor SECOND SERIES, Vol. XIX, No. 56.-March, 1855.

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