on with appliances of a very primitive character. "The instruments used," writes Dr. W. Pryce in 1778, are, a compass without a gnomon or style, but a center pin projecting from the middle of the compass to loop a line to, or stick a candle upon, fixed in a box exactly true and level with its surface, about 6, 8, or 9 inches square, nicely glazed with strong white glass, and a cover suitable to it hung square and level with the upper part of the instrument: a twenty-four inch gauge or two-foot rule, and a string or small cord with a plummet at the end of it: a little stool, to place the dial horizontally: and pegs and pins of wood, a piece of chalk, and pen, ink, and paper."

The author warns "those who take no account of the points or angles of the compass, but in lieu thereof, chalk the bearing of the line they measure with, on the board the compass lies in; for if they are not exceedingly careful and precise in their operations, they may commit almost unpardonable and irretrievable blunders : yet formerly, before penmanship and figures were so generally understood and practised among the common Tinners, as they are at present, most of our Mines and Adits were dialled for in this manner."

Towards the end of the last century, the dial was fitted with sights, by means of which the direction of the station-line could be taken with precision.

Description of the Miner's Dial.-In its simplest form, the miner's dial consists of a box of brass or wood, on the base of which is fixed a brass ring divided into 360°. The base of the box within this ring is also graduated, but each division contains 10°, and the numbers proceed from the north and south points to 90° on each side, thus dividing the inner circle into four quadrants of 90° each. On a finely pointed pin fixed in the centre of the circle, a magnetic-needle is freely suspended, so that when the dial is placed in a horizontal position and the needle unchecked, one end points towards the north. This north-seeking end is distinguished by some mark. The instrument is supplied with a glass cover, and also with a brass lid to protect it when not in use. Perpendicular to the horizontal plane of the instrument, are two brass plates, called sights, one at the north, and the other at the south point. These sight vanes are divided into two parts, the upper one on one side having a fine slit cut throughout. The corresponding division of the opposite sight carries a plain wire. In the lower divisions, the relative positions of the slit and wire are changed. The compass-box is attached to a tripodstand by a socket fitting on to a corresponding plug; an intermediate ball and socket joint furnishing the means of levelling the instrument.

(a.) The Magnetic-Needle.-In shape, the magnetic-needle is usually rhombic (Fig. 8) or rectangular (Fig. 9), or its height may be greater than its breadth, in which case the edges are bevelled (Fig. 10). Magnetic-needles of the rhombic form have the advantage of lightness. It is, however, not advisable to make the needle pointed, as it retains its magnetism longer when the ends are square. In the form shown in Fig. 9, the points are replaced by a fine etched line, representing the magnetic axis.

Fig. 8.

Fig. 9.

Fig. 10.

The needle is drilled through in the centre, and carries above the aperture a hollow brass cap, lined with some hard stone conically hollowed out. Agate or carnelian is usually employed for the purpose; but ruby is best. The cap must be as light as possible, and it must be firmly fixed to the needle in such a way that its axis forms a right angle with the axis of the needle. The interior of the cap must be accurately conical. Caps made of brass, silver, or steel should be avoided, as they cannot be polished so well as those of agate, and they are very soon bored through by the centre pin. This pin is made of good steel with a hard, smooth, round point, the angle of which is not too obtuse. The more pointed the pin and the more conical the interior of the cap, the less is the friction of the needle on its pivot.

The needle must be made of pure spring-steel, and so constructed that its geometrical centre line passes exactly through the centre of the cap. It must be sufficiently magnetised, and, when placed on its pivot, must assume a horizontal position. A needle which is horizontal before being magnetised, will dip after having been subjected to that treatment. It is, therefore, necessary to make one end of the needle heavier than the other.

In order to preserve the pin from unnecessary wear, and from being broken off when the instrument is carried, a contrivance is employed for fixing the needle. This consists of a slide, pressed from outside, which raises the needle and presses it against the glass lid of the dial. When required to be used, the needle should be lowered carefully, so that it gently rests and does not fall upon the centre pin.

A

Much depends upon the sensitiveness of the needle. sluggish needle is utterly useless. The needle may be tested by bringing a piece of iron near it when at rest, observing whether it returns exactly to its former position after a few oscillations.

The test should be made at several points round the dial. The needle should not move when the dial is gently revolved.

All parts of the dial, with the exception of the magnetic-needle and the centre pin, must be made of metal free from iron or nickel. They may be tested by bringing them near a sensitive magnetic-needle balanced on a centre pin fixed in a piece of wood, and noting whether the needle moves, as each separate portion of the dial is brought near it.

In making a survey with the dial, care must be taken that nothing capable of attracting the needle is carried about the person, such as penknives, keys, steel watch-chains, spectacles, nickel-plated studs, or iron rivets in the magnifier used to read the graduations. Watches in which the movements are made of nickel, attract the needle almost as much as when the movements are made of iron. The brims of felt hats are sometimes stiffened by inserting an iron wire round the edge. The surveyor should therefore examine both his watch and his hat before commencing a survey. Sometimes the needle persistently sticks to the under side of the glass. This is caused by the glass becoming electrified from rubbing against the clothes, or from being cleaned with a silk handkerchief. The electricity may be at once removed by touching the glass with the moistened finger or by breathing on the glass. An unsuspected source of error in magnetic-needle readings has recently been discovered to arise from the magnifying glass used for reading the graduations becoming electrified. The magnifier generally used for that purpose has a hard, highly-polished, black frame, which is peculiarly liable to become electrified, even by the mere carrying in the pocket, so that, when brought near the magnetic-needle, it draws it sometimes as much as half a degree from its true resting place.

In a good dial the centre pin of the needle should be exactly in the centre of the graduated circle, and the needle should be straight. If this is not the case, there will be an error of eccentricity in every observation. The constant eccentricity, when the centre pin and the ends of the needle are not in one vertical plane, and the variable eccentricity, when the centre pin is eccentric to the graduated circle, may be detected by the readings at the two ends of the needle not agreeing. In both cases the error may be corrected by reading both ends of the needle, and by taking the mean of the two results. Cases of irregular eccentricity are sometimes met with that is, when the point of suspension of the needle in the cap is variable. Needles presenting this error are useless.

(b.) Spirit-Levels. On the dial-plate are two small spirit-levels, consisting of glass tubes slightly curved and nearly filled with

some limpid liquid, a bubble of air being left. One of the spiritlevels is parallel to the direction of the sights, whilst the other is at right angles to it. They are so adjusted that when the bubbles are in the centres of the tubes, the dial is level.

Spirit-levels are usually filled with alcohol or ether. The bubble, being specifically lighter than the liquid, always assumes the highest possible position; and if the tube has been ground to a perfectly circular longitudinal section, the tangent to its inner surface at the centre of the air bubble is a horizontal line.

(c.) The Tripod.-The dial is usually supported on three legs, shod with iron, and connected at the top in such a way that they are movable in any direction, lightness and rigidity being the qualities desired. Usually the legs are made with a screw-joint in the middle, and a set of extra points is provided to screw on, when the workings are low. The dial is connected with the tripod by means of a ball and socket joint, which consists of a ball at the end of a covered spindle fitting into a corresponding cavity under the dial-plate. The ball turns in a socket, and can be loosened or tightened at will. It thus admits of motion in any direction.

(1.) Taking Underground Observations with the Dial.-The bearing of a line is the angle which it forms with the direction of the magnetic-needle. To take the bearing of any line, set the compass exactly over any point in the line by means of a plumbline suspended from beneath the centre of the dial; level the instrument, and direct the sights to an object at the other end of the line. Then measure the line, and note the distance measured in the dialling-book. The needle will thus have been allowed sufficient time to come to rest. A second look along the line may now be taken in order to test the accuracy of the observation, the eye being applied to the south sight. The number of degrees to which the north-seeking end of the needle points is then carefully noted. This method of taking an observation with the north sight in advance is that generally employed. The results are called fore-observations. Sometimes, however, it is desirable to place the eye at the north sight, and look back in a direction contrary to the order of the survey. Observations taken in this way are called back-observations. The angle is read from the north-seeking end of the needle, and entered in the dialling-book as if it had been a fore-observation. This method is employed in dialling a line from the centre of a shaft, where the instrument cannot conveniently be set up. By using the back-observation throughout a survey, taking back-observations and fore-observations alternately, the instrument is moved only half the number of times it would otherwise be.

The lettering of the miner's dial differs in an important par ticular from that of the mariner's compass or pocket geological compass. When we face the north, the east point is on our right hand, and the west on our left, and the graduated card of the mariner's compass is marked accordingly. In the miner's dial, however, the letters representing east and west are transposed.

Fig. 11.

The reason of this will be seen from Fig. 11.
Here the dial is represented with the east and
west points in their true position. The line of
sight in which the observation is taken lies
over the north and south line marked N S.
This is to be placed in any required direction,
and being fixed, the magnetic-needle is found.
to rest, we will suppose, in the position indi-
cated by the dotted line AB, the north-
seeking end of the needle coming to rest at 24°
distant from the north and south line. The
reading of this is not N. 24° W., as might at
first be supposed, but N. 24° E.
The reason

is apparent on considering that the needle is the only representative of the magnetic bearing. If then a corresponding line C D is drawn upon paper, the end C will represent the magnetic north. The line N S coincides with a line in the direction of the road to be surveyed, which on the plan will be represented by a line EF parallel to N S, and this line of direction is clearly seen to be on the east side of the magnetic meridian, forming with it an angle of 24°

In order to prevent confusion, miner's dials are, as a rule, graduated from right to left, the east and west points being transposed. An illustration of the necessity of this change may be afforded by imagining a watch in which the dial-plate moves from left to right, whilst the hand remains immovable. It is evident that the hours must count from right to left for the watch to indicate the right time. Similarly, in the case of the miner's dial, the magnetic-needle always points to the north, and may consequently be compared with the fixed hand of the supposed watch.

In some of the older patterns of miner's dial the letters are not transposed, the east being on the right and the west on the left. In using a dial of this kind, the letters must be mentally reversed before the bearing is noted. The best method to adopt for entirely avoiding all confusion is to disregard the lettering of the dial, and read the azimuth or meridian angle, remembering that 90° represents the east, 270° west, and 180° south.

Taking Vertical Angles. In the old type of miner's dial, the