By looking along one which exactly It will be noticed

number of minutes by means of the vernier. the vernier graduations it is found that the coincides with a limb graduation is marked a. that the coinciding graduation is on the right hand vernier, and as the vernier has been moved from the zero of the limb to the right, it is the proceeding vernier, therefore the lower set of figures of the vernier are read. Counting from the vernier zero it is found that the coinciding graduation is the fourth, and as each division indicates three minutes the reading is twelve minutes. These twelve minutes must now be added to the five degrees previously read, giving as a final reading five degrees twelve minutes.

In Fig. 83 it will be seen that the vernier has again been moved to the right, as this is the direction of the increase in the

40

FIG. 83.

numbers of the limb graduations. The reading on the limb opposite the zero of the vernier is between 39 and 40, or 39+ a number of minutes which will be determined with the vernier. By looking along the graduations of the vernier until a coinciding line is found, it will be noticed that it is the one marked a, and that it is on the left hand vernier, or what in this case is the backward vernier. The reading is therefore taken from the top row of figures which read from the end towards the centre. The coinciding graduation is one nearer the end than that marked 45, and as the numbers decrease in this direction, and each division equals three minutes, the reading is 45-3=42 minutes. This added to 39 degrees gives as a final reading 39 degrees 42 minutes.

Taking Fig. 84 next into consideration, it is perceived that the vernier has been moved to the left, and that the limb reading opposite the vernier zero is between 39 and 40, or 39+ a number of minutes which will be found with the vernier. Looking along the vernier graduations as before, the coinciding

line is seen to be that marked a, and that it is on the left hand or proceeding vernier. We then know that the reading of the vernier must be taken from the lower set of figures, and see that

it is two graduations or six minutes past the fifteen minutes' division, therefore the reading is 21 minutes.

is therefore 39 degrees 21 minutes.

The final reading

The limb reading and 26, or 25+ a The coinciding vernier

In Fig. 85 the vernier is again to the left. opposite the vernier zero is between 25 number of minutes yet to be determined.

graduation is a, and being on the right hand or backward part of the vernier, the vernier reading must be taken from the top row of figures. The "a" graduation is two divisions, or six minutes from the outer edge, which gives as the vernier reading 30+ 6 = 36 minutes. This added to the limb reading gives a total of 25 degrees 36 minutes.

CHAPTER X

SURVEYING INSTRUMENTS

The Hedley Dial.-A good idea of the construction of the miners' dial can be formed by reference to the accompanying

FIG. 86.

illustrations. Fig. 86 shows the ordinary appearance of the miners' dial. Fig. 87 shows the appearance when taking sights in mines of steep inclination with an arc attached, for taking

dip. Fig. 88 is a view of the face or front of the dial, and Fig. 89 the back view.

It consists of a shallow cylindrical brass box on the inside of which is a raised ring, which is graduated into the degrees of a

FIG. 87.

circle, viz. 360. On this circle the numbers are marked consecutively from 0 to 360, but on another circle at the base of the box the graduations are in tens, and are numbered from the north and south of the dial towards the west and east on both sides, thus forming four quadrants of 90 degrees each. the centre of the box is a vertical finely-pointed pin, which supports a magnetic needle. The needle is slightly shorter than the inner diameter of the raised ring, and its height when swinging freely is such that the top of the needle is on a level with the raised ring, thus enabling the position of the needle in relation to the graduations to be read off accurately.

A vernier is connected rigidly to the inside of the box,

F

immediately above the graduated circle at the north end of the dial, and by removing the peg (c Fig. 89), the box with the vernier, etc., can be moved round the circles.

The cap (a, Fig. 89) fits on a tripod stand which has a ball and socket joint to enable the instrument to be levelled. The inner circles are connected rigidly on the cylindrical piece of metal which forms the cap of the legs, and if the dial be first