On a New Induction Dip-Circle. By JOHN ALLAN BROUN, F.R.S. The idea of determining the earth's magnetic intensity by its inducing action on soft iron was employed by Dr. Lloyd for the purpose of obtaining the magnetic inclination. A soft iron bar being placed vertically, so that the induced magnetism of one end should act on a freely suspended magnet, the deflection thus produced was observed, and considered proportional to the vertical component of the earth's magnetic intensity; the bar was then placed horizontal, and, the same end acting, the deflection was observed, which was in the same way considered proportional to the horizontal component: were there no sources of error, the inclination might be determined from these two angles. The iron bars employed always possess or acquire a certain amount of induced magnetism, the effect of which is eliminated by inverting the bar for the different deflections; there are, however, still two sources of error which remain. The most important is that due to the different actions of the different parts of the bar in the vertical and horizontal positions. If the whole magnetism were accumulated in one point at the acting end of the bar, this source of error would not have existed; but as the magnetism is distributed over the whole length, that part whose action is equal on both ends of the suspended magnet when the bar is in the vertical position, becomes greater on one end of the magnet than the other when the bar is in the horizontal position. It was probably for this reason that Dr. Lloyd's method has never been put into practice. Last year, while observing with Dr. Lamont's theodolite magnetometer, Mr. Broun employed a method for the determination of the absolute magnetic inclination, to which it is believed there can be no objection in low magnetic latitudes, and which, with the modifications proposed, may probably be used in all latitudes. In Dr. Lamont's apparatus the variations of magnetic dip from place to place are determined by means of two soft iron bars clamped to a horizontal ring, the ring surrounding a freely suspended magnet, one bar vertically above the ring, the other vertically below it. By a series of observations of the deflections produced by the bars in different positions, inverted and exchanged from side to side, the effect of permanent magnetism is eliminated, and the deflection due to the earth's force is obtained; the sine of this angle, multiplied by a constant, gives the dip for each place; the constant, however, requires the aid of the usual dip apparatus for its determination. It is evident, however, that if we can incline the bars moving in the plane of the magnetic meridian till the observed deflection be zero (should there be no permanent magnetism), and observe the angle through which the bars have been moved from the vertical, this angle will evidently be that of the magnetic inclination, for the bar will have been moved into the direction at right angles to that of the total force. This method, as thus stated, requires the determination of the vertical position of the bars; and it is supposed that there is no permanent magnetism: as far as the latter supposition is concerned, the error is eliminated by reversing the bars; in order to render the determination of the vertical position unnecessary, it is only required to observe the angular inclination of the bars, which (for each position) diminishes the deflection by an amount equal to the mean deflection previously obtained. It will be observed that for low latitudes, where the bars are moved little from the vertical, the objection applying to Dr. Lloyd's method exists to so small a degree as to be negligible. This method, which Mr. Broun employed in India, is, however, liable to error in high magnetic latitudes; and the following is proposed for use in all positions. A small magnet, 2 inches long, is suspended by a silk fibre as with the usual declination magnet; a small mirror attached to the magnet allows the determination of the magnetic meridian by means of a telescope having a prism near the wire at the eyepiece, as in Dr. Lamont's apparatus. When the wire coincides with its image reflected by the mirror (no disturbing cause being near), the magnet is in the magnetic meridian. A vertical circle in the magnetic meridian parallel to the magnet, and 3 inches distant, centre to centre, has a soft iron bar clamped to the alidade, so that the acting pole of the bar is opposite the centre of the circle and the middle of the magnet. The reading of the circle is first obtained for the bar vertical: the verticality of the axis of the bar may be determined in different manners; the best, perhaps, is to have the bar hollow, and to employ reflexion of a cross wire from a surface of mercury. The bar is then moved in the magnetic meridian from the vertical position till the deflection of the magnet is zero; if the permanent magnetism acts with the induced magnetism, the movement of the bar will be greater than the inclination by a given angle; in turning the bar in the opposite direction (so as to invert it), the angle from the vertical will be less by the same amount. Since in the position at right angles to the magnetic force the induced magnetism is zero, the objection applying to Dr. Lloyd's method does not exist; there is, however, still a source of error remaining that applies to both: as the magnetic inclination increases, the position at right angles to the force can only be attained by moving the bar nearer and nearer to the horizontal, and as it approaches the horizontal, a certain amount of magnetism is induced in the bar by the small suspended magnet. Different methods have been imagined by the author to destroy or balance this action; but the best method he thinks will be to make observations with the bar at two different distances. The magnetism induced by the small magnet in the bar may be represented by a weak magnet, whose force will vary inversely as the cube of the distance: as the action of this weak magnet will also vary inversely as the cube of the distance, the effect may be determined and eliminated by observations at two or more distances. Any error of the observation for the vertical position of the bar due to the noncoincidence of the axis of magnetism and of figure may be eliminated by turning the bar on its vertical axis of figure through 180°. The author remarked that the error due to the inducing action of the small suspended magnet might be rendered as small as we please, by employing a modification of the method used by him in India. If the total deflection due to the bar vertical (direct and inverted) be determined, and we then observe the change of deflection due to a given angular movement of the bar from the vertical, we may compute the movement necessary to render the deflection zero: the angular movement may be taken of such magnitude as to render the effect of the inducing action negligible. This modification requires, however, the determination of the angles of deflection, and therefore is far from the simplicity of the first method. The author pointed out, that, since when the bar is at right angles to the direction of the total force any small movement of the bar will produce induced magnetism in proportion to the sine of the small angle of movement, this position is that best fitted to give the true position of the magnetic meridian with the least error of inclination. The author concluded by stating that he had learned since his return to Europe that Dr. Lamont had also proposed a method differing from that of Dr. Lloyd. Dr. Lamont employed an astatic needle, and turned the bars into different azimuths by movement on a vertical axis, so as to produce different amounts of induced magnetism without changing the position of the bars relatively to the vertical. This method, Dr. Lamont informed the author, had failed on account of the bars receiving different amounts of permanent magnetism in changing from azimuth to azimuth. This difficulty does not exist in Mr. Broun's method, as the bar is always kept in the meridian, and is always brought to the position where the inducing action is zero. On Magnetic Rocks in South India. By JOHN ALLAN BROUN, F.R.S. The Moocoonoomalley is a granite hill rising about 800 feet above the sea, 5 miles south-east of Trevandrum, and about 35 miles north-west of Cape Comorin. General Cullen, the late British Minister at Travancore, had observed several anomalies in the magnetic dip in ascending this hill. The dip near Trevandrum and about the base of the hill was from 2° 30' to 2° 40' S.; on the top he found the dip to be from 5° 52′ to 11° 23′ in different years, in which he probably slightly varied the position of observation. In December 1855 I examined the rock masses constituting the hill. The plain around the base is formed of a stratified rock known to Indian geologists by the name of laterite. The first rocks in the ascent are dark syenites, containing a considerable proportion of hornblende (in some cases the appearance is more like a greenstone); towards the middle of the ascent light-grey syenites become common, and at the top the rocks are pegmatites or granites. I first examined a small fragment of the rock presented to me by General Cullen, of a greyish-red tint, composed chiefly of felspar and quartz with particles of magnetic iron ore disseminated; these particles were of about to inch in diameter, and without any regular form or smooth face (as far as my examination went), when a magnet was presented to one of these particles, detached from the specimen, it showed its polarity by tumbling over, if the homonymous pole was at first nearest the magnet. The specimen alluded to was about 5 inches long, 24 inches broad, and 1 inch thick, tapering and thinning off to one end (A). On presenting the different extremities to a freely suspended magnet (the declination magnet of the Trevandrum Observatory), the following results were obtained :— where the negative sign signifies repulsion of the north end of the magnet, and the positive sign attraction of the same pole. The changes of magnetic declination occurring during the experiments were observed by another instrument, and have been subducted. As the line of the magnetic axis of the specimen was evidently towards the direction of its greatest length, the northern end being towards A and C, it was desired to determine whether the same relation would hold true for any fragment; for this purpose, two ends of the specimen were knocked off, leaving a fragment in the middle with a distance of a (towards A) to b (towards B) of nearly 2 inches, while the breadth from C to D was nearly 3 inches; so that the longest dimension was now nearly at right angles to that of the whole specimen. The central fragment being placed at the same distance from the suspended magnet as in the previous experiment, the following were the results : The ends and sides show the same polarities as in the whole specimen, but with the north end of the magnetic axis turned more to the side D, for which the deflection has increased. Upon presenting the small fragment constituting the end A of the specimen, the results were as follows: Sc. div. 0.00 -0.36 +0.36 +0.14? +0.14 Here it will be observed that the broken fragments of the specimen acted exactly as the broken parts of a magnet; thus the end a in the central fragment gave a repulsive effect of 0.79 scale divisions, while the end a', the opposite face of the fracture, gave an attractive action of 0.36. Several questions of interest presented themselves in connexion with these rocks. Whether the hill as a whole would give results similar to those obtained from this specimen? Whether the lines of magnetic force in it had any relation to the lines of crystallization, or to those of the earth's poles? Whether any particular direction was most favoured? or whether the magnetic axes vary from spot to spot, and the magneticules, possessing their present magnetism when tossed up in the liquid mass, had their positions determined by chance? On the 11th of December, 1855. I visited the hill, making observations with a 6-inch 26 dip-circle by Robinson; the following differential results were obtained without reversing the poles of needle: : 4 P.M. Base of hill over laterite.... 1 50 1 55 The bearing of the Trevandrum Observatory was observed approximately by a hand compass at the stations A and B. At 4 feet above the rock the error was small; but when placed on the rock A, the declination was found 10° west, while on B (9 feet W.N.W. of A) it was 35° east, the true declination over laterite being about east. The pegmatite and granite on the top of the hill seemed to form kinds of dykes running parallel to each other nearly north and south, and crossed by lines nearly at Blocks of about 9 inches right angles to the direction, so as to form large blocks, between which the decaying rock has allowed the accumulation of soil to some depth. diameter were cut out of the rock at A and B, having previously marked the direction of the true north and south upon the upper surface, which was nearly horizontal. With these specimens I made the following observations. A specimen was placed with its centre at about 2 feet from the centre of the freely suspended magnet, and in the line at right angles to the direction of the magnet; the points of the compass marked on the upper surface, when the specimen was in situ, were successively presented to the centre of the magnet, and the scale readings of the instrument were observed. The direction of the plane of greatest force being found, the specimen was inclined at different angles to the horizontal, till the direction of the line of greatest force was determined. Specimen A: elliptic cylinder, axes 9 inches and 8 inches, average height about 5 inches; a granite containing a small quantity of hornblende, colour reddish grey; from about 1 foot west of the position A for the dip observation. The numbers following are in scale divisions, each equal 15" nearly : +21.4 S. deflection ... " +19.1 S.S.W. " +15.3 S.W. -17.8 E. + 0.8 2.3 S.E. -12.3 S.S.E. -15.4 N.W. +14.5 " +19.9 +21.4 The direction of the plane containing the magnetic axis is in this case nearly north and south. On raising the point S. presented to the magnet, it was found that the north end of the magnetic axis dipped from 10° to 20° below the south horizon; the exact position could not be determined, from the difficulty of keeping the centre of the specimen always at the same distance from the magnet. The result agrees with the fact that the dip observed on A was diminished, since the rock magnet having here its south end uppermost, would necessarily attract the north end of the dipping needle. Specimen B: cylinder 10 inches diameter, 9 inches deep; upper surface red and weathered, interior bluish grey; contains besides the bluish felspar and quartz a large quantity of hornblende. The observations for B were made only for the north end of the axis. The north end of the magnetic axis was here evidently nearly in the direction N. by E. E. Upon raising this point of the stone, presented to the centre of the magnet, the deflection diminished; on lowering it, the deflection increased to 10°; su that the north end of the axis here inclined 10° above the horizon to N. by E. E. This result also agrees with the increase of dip found at B. In a third specimen examined, which was weakly magnetic, the north end of the axis made an angle of 80° above the N.W. by W. point of the horizon. From these results it is evident that though the direction of the magnetic axis may not vary much in small specimens, it does so in parts of the rock separated by a few feet only from each other; and it appears probable that it may be considerable for smaller distances than those under experiment. Neither do the directions of the axis seem to have any relation to the lines of crystallization. Another question was examined by me, namely, whether the magnetic intensity of the rock varied with the temperature. For this question I chose a specimen of about 6 inches long by 4 broad and 3 thick, taken from near the middle of the ascent of the hill. The observations were made in the same manner as for the temperature coefficient of a magnet. The specimen was placed in a wooden trough, into which water of different temperatures was poured: the deflections of the declination magnet by the specimen at different temperatures were noted; the variations of declination during the experiments were eliminated by means of another instrument. The results are contained in the following Table : The result is that the magnetic rock, like a steel magnet, loses force by an increase of temperature; and, using the notation employed for steel magnets, the temperature coefficient is approximately q=0'000214, nearly the value obtained for steel magnets used in the British and Colonial Observatories. The following may be considered as the conclusions at which I have arrived :1st. The rock fragments have determinate magnetic axes. 2nd. Broken fragments resemble broken magnets, showing opposite polarities at the two surfaces of fracture. 3rd. The magnetic axis varies from place to place within small distances. 4th. The action of the whole hill on magnets freely suspended at moderate distances is nearly imperceptible; the opposite directions of the magnetic axis in the rocks rendering the total action nearly zero. 5th. As in some cases the north end of the magnetic axis was found to the southward (as with specimen B), we cannot suppose that the magnetism of the small magnets has been due to the inducing action of the earth in their present position or since the rock mass became solid. 6th. The directions of the magnetic axis have no relation to the lines of division of the rock masses. 7th. The magnetic force of the rock masses varies with temperature like that of steel magnets. On a Magnetic Survey of the West Coast of India. By JOHN ALLAN BROUN, F.R.S. This survey was undertaken at the expense of His Highness the Rajah of Travan |