When the deviations are caused by the soft iron rod instead of by the magnet, somewhat similar formulæ are obtained; but the magnetism of the iron rod being due to induction, its intensity is proportional to the variations of the vertical components of the earth's magnetism. It follows that the constant K of each formula in this case must be replaced by a magnitude that varies with the magnetism of the rod. Observations with the iron rod indicate the inclination of the earth's magnetism; whilst observations with the bar magnet serve for determining the horizontal components of the same terrestrial force. Consequently, by combining the two methods, it is possible to find out the vertical components of the magnetic force.

In order to survey an ore field, it must first be divided into squares with sides 100, 50, or 25 feet in length. Then at every angle of these squares, the deviation must be observed with the magnet and iron rod. Similar observations must be made on ground free from iron, and so far distant from the ore field that the influence of the ore is not felt. It is also advisable to determine the magnetic declination for each point of observation. This may be done by directing the sights along one of the lines that have been set out, and reading the bearing, after the fixed magnet and iron rod have been removed. Observations must also be made along the magnetic meridian north of the supposed ore pole to determine where the north-seeking end of the free . needle changes its direction from north to south, or whether it invariably points towards the north.

When these determinations of declination, horizontal intensity, and inclination have been carefully made, and the angles obtained noted on paper divided into squares, lines are drawn for each of the three series of observations, exhibiting equal declination (isogonic lines), equal intensity (isodynamic lines), and equal inclination (isoclinic lines). This is done in each case by joining the points by which equal angles were obtained. The curvature of the lines is drawn as naturally as possible, care being taken to avoid sharp bends. The curves of inclination and intensity thus constructed are closed, and have an approximately circular or elliptical shape, provided that a single isolated ore mass is being dealt with. They are grouped round two points. The one at the north is where the greatest angle of deviation was found, whilst that at the south is where the smallest angle was obtained. Between these two groups of curves is an open curved line representing the neutral angle. In this neutral line the intensity is the same as if no ore was present. The straight line joining the points where the greatest and smallest angles were obtained passes over the centre of the

ore mass, and indicates the direction of the magnetic meridian of the ore field. Directly beneath a point in this line, in a vertical ore bed, the greatest mass of ore occurs. The rule that most generally holds good in searching for iron ore is, that the ore mass is to be found immediately beneath the point where the magnetic meridian cuts the neutral line.

The isogonic lines consist of concentric ovals placed, as a rule, symmetrically on both sides of the meridian. From the shape and position of these curves useful indications may be obtained regarding the position of the ore pole, and the shape of the deposit.

3. Tiberg's Method. In exploring for iron ore, Mr. E. Tiberg uses a dip-compass, 31 inches in diameter, and half an inch deep. The axis of the needle is at right angles to the plane of the box, and rests upon two agate supports. The needle can thus move freely when the compass is placed horizontally or vertically. The instrument differs from other dipping needles in that the centre of gravity of the magnetic-needle is a little below its horizontal axis when the compass is in a vertical position. The needle is compensated for the vertical force of the earth's magnetism by a piece of wax fastened to its southseeking end.

The instrument is provided with a spirit-level for horizontal adjustment, and with a ring, by means of which it can be suspended vertically. The sighting instrument, used in conjunction with the dip-compass, is a brass plate about a foot in length, provided at one end with four square flanges to receive the dipcompass for horizontal measurements. At right angles to this square, there is a groove in the plate with a sliding receptacle for the bar magnet required for horizontal measurements. Four folding sights are attached to the plate in such a way that their lines of sight form a right angle. The instrument, consequently, can be used as a cross-head. Two special sights are added for levelling operations, and the instrument is provided with a circular spirit-level.

The observations for vertical measurements are made at the surface with the plane-table or by hand. The inclination instrument is fastened to the plane-table, levelled, and turned until the needle points to 90°. The instrument is then raised with the ring at the top, and placed at right angles to the magnetic meridian, and the angle indicated by the needle observed. The same operation has to be done by hand if the plane-table is not available. When the ore appears to be deep, or when the horizontal intensity is powerful, recourse must be had to the plane-table.

The formula for calculating the vertical intensity G is—

G=K tan v,

in which v is the angle given by the needle—that is, its deviation from the horizontal—and K a constant varying in different instruments from 0·75 to 14 of the earth's horizontal magnetic force. Lines of equal vertical intensity may thus be constructed. In magnetic plans it is usual to employ a blue colour for positive intensity, and a red colour for negative intensity. The accuracy attainable with this method is from 0.2 to 0.1 per cent. of the earth's magnetic force in central Sweden. With the plane-table 250 to 300 observations may be made per day, and 450 to 500 by hand. For each ore field surveyed the needle must be compensated afresh, and a preliminary magnetic survey made. The field is then divided into squares, with sides 40 feet in length. The base-line is as nearly as possible in the middle of the field, and parallel to the direction of the strike of the deposit. In making the survey, observations are made every 10 feet, and in some cases every 5 feet, in the immediate vicinity of the ore, and every 20 to 40 feet or more when farther distant from the ore. The general rule is to make as many observations as may be required to indicate what the appearance of the curves will be. Heights are estimated by the eye, or by a preliminary levelling with the sighting instrument, and the more important topographical details are noted.

The maximum of intensity is generally presented by the point where the ore is nearest to the surface. It may also be situated between two adjacent deposits-in which case the intensity decreases, at first slowly, or not at all, and then comparatively rapidly. The distance to the centre of a vertical ore bed may be taken as at least 0.7 of half the breadth of the north-polar attraction. This rule is, however, not very trustworthy. The vertical distance of the plane of observation from the upper ore pole is equal to the horizontal distance of the point where the needle deviated most from the horizontal from that where of the greatest intensity was found. It is also equal to 13 of the distance of the point where the needle dipped most from that where half the maximum was found. The latter rule is the best.

Sometimes these calculations enable an opinion to be formed of the relative values of two similar ore beds. For two deposits of a similar character, situated at least 30 feet beneath the surface, it may be assumed that the deposit, for which the product of the greatest intensity and the polar distance is the greater, contains the larger quantity of ore for the same length of deposit. If the polar surfaces of the two beds are limited this product must be replaced by the square of the polar distance.

A good idea of a deposit may be formed from the appearance of the curves of intensity. Regular, long extended, elliptical curves, enclosing a long but narrow district of greatest intensity, always indicate a regular lenticular mass. An asymmetrical bend in the curves indicates parallel deposits. More circular curves may indicate a segregation of ore if the intensity decreases regularly. Irregular curves indicate more or less irregular deposits.

In exploring for courses of ore in the mine, a base-line is marked out in the level, and observations made every 10 feet at least. At each station three observations have to be made :— (1) To determine the direction of the total horizontal intensity by means of the sighting instrument, the deviation of the magnetic-needle from the base-line being observed. (2) To determine the magnitude of its force by means of the bar magnet. (3) To determine the vertical intensity by means of the inclination instrument. Vertical measurements must also be made at the top and floor of the level, and for this purpose the instrument may be held in the hand. On neutral ground at the surface, the horizontal force of the earth's magnetism and the direction of the earth's magnetic meridian must be determined. The results of all the observations are represented on paper, along the base-line, as arrows showing the horizontal forces of the magnetism of the ore at the points of observation. If all or part of the arrows are directed towards the same point, there the ore may be assumed to be. The ore would be at the level at which the observations were made, if the vertical intensity is negative. When the arrows approach in front or behind, the plane of observation is above or below the magnetic centre of the ore. When the vertical intensity is positive, the ore may be above or below the plane of observation, always assuming that a more or less vertical ore mass is being dealt with.

By applying this method Mr. Tiberg has discovered important deposits of ore at the Swedish mines of Langban and Sikberg.

To illustrate the value of the magnetic-needle in exploring for iron ore, it may be mentioned that, according to the statistics given by Prof. Smock,* there were 115 mines in 1868 in the State of New Jersey, whilst in 1874 the number had increased to 200. All these ore localities were first made known by the use of the magnetic-needle. In fact, the annual production of the State had been increased 50 per cent. by the addition of new producing localities found by the compass. It should also be stated that in many cases there are no visible surface indications of ore.

* Trans. Amer. Inst. M.E., vol. iv., 1876, p. 353.

Use of the Magnetic-Needle in Surveying Bore-holes.—It has been assumed that the diamond drill always bores a perfectly straight hole, even though passing through rocks of different hardness. Actual experience reveals an entirely different state of things, the deviations sometimes being so great as to render a bore-hole misleading. An ingenious plan of correctly ascertaining these deviations has been devised by Mr. E. F. Macgeorge,* an Australian engineer. His plan consists in lowering into the bore-hole clear glass phials filled with a hot solution of gelatine, each containing, in suspension, a magnetic-needle, free to assume the meridian direction. The phials are encased in a brass protecting tube, and let down to the depth required, being allowed to remain for several hours until the gelatine has set.

0

It

The construction of the phials or clinostats can be seen from Fig. 96. The clinostat is a true cylinder of glass made to fit accurately within the brass guide-tube. At the lower end it terminates in a short neck and bulb, within which a magnetic-needle is so held by a glass float as to stand upright upon its pivot in every position of the phial, and thus allow the needle to assume the meridian freely without touching the sides of the bulb. Passed through an air-tight cork and screw-capsule at the upper end is a small glass tube terminating in another bulb above and with its open lower end inserted in a cork which enters the lower neck of the phial, thus preventing the escape of the needle and float in the lower bulb. The upper Fig. 96. bulb contains a very delicate plumb-rod of glass consisting of a fine rod terminating in a plumb of glass below and a diminutive bulbous float of hollow glass above. is carefully adjusted to the specific gravity of the gelatine in which it is immersed, so as to insure the rod being truly vertical whatever the position of the phial and bulb may be. When the gelatine is fluid the plummet hangs freely perpendicular, whilst the needle in the lower bulb assumes the magnetic meridian. When, however, the phial is at rest in any position, the contents solidify on cooling, and thus hold fast the indicating plummet and magnet in solid transparent material. On withdrawal from the bore-hole the phials can each be replaced at the same angle at which they cooled, and when the phial is revolved upon the part where the magnetic-needle is seen embedded in the gelatine, until the needle is again in the meridian, the phial is manifestly in the same direction, both as regards inclination and azimuth, as it was when its contents were congealed, and thus the gradient and bearings of the bore-hole can * Engineering, vol. xxxix., 1885, pp. 260, 334.