hold it there until two opposite screws are put in place and screwed into the ring. Now pull the stick out of the screw-hole of the diaphragm, and with it turn the diaphragm about the two screws already screwed into the ring until the other two screw-holes are in their proper position. Screw in the other two screws, and adjust the diaphragm for verticality of the cross hairs and for collimation. Glass diaphragms will save any of this trouble with cross hairs.

To Replace a Spirit Level or Bubble Glass.-Remove the level from the instrument, pull off its sliding ends, and take out the broken glass. Put in the new one with the graduated side up. Roll some paper round its ends if it fits loosely. Putty or melted beeswax round the ends of the glass will hold it firmly in its tube.

Adjustment of the Sextant-For adjustments of the sextant, see Chapter X.

Adjustment of the Box Sextant.-When the box sextant is in adjustment both the mirrors are at right angles to the plane of the instrument, i.e., the top of the box, and when the vernier is at zero are parallel to each other. The index glass is permanently fixed by the maker. The horizon glass is adjusted as follows:-Bring the zero of the vernier to the zero of the graduated arc, and look through the eyehole and the unsilvered half of the horizon glass at some distant object. If the instrument is in adjustment this object and its reflection will appear to coincide exactly. If not, the two will appear to be separated either horizontally or vertically, or both, as In this case, apply the key furnished with the instrument to the square-headed screw in the top of the box, and bring the object and its image into a horizontal position, thus **. Then apply the key to the other square-headed screw in the side of the box, and bring the object and its image to coincide exactly. The instrument will then be in adjustment. See also adjustment of sextant, Chapter X.

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Adjustment of the Compass.-The adjustments of the compass are as follows:

To make the Needle truly Horizontal.-Level the compass, and then see if the needle is level. If not, make it level by moving the wire which is fastened round it towards the high end.

To make the Sights Perpendicular to the Compass Plate. Hang up a plumb line, and having levelled up the compass sight on to it, see if the slits coincide with it. If the slits should not be in adjustment, they may be rectified by unscrewing the sights, and filing off a portion of the feet on the high side, otherwise they may be wedged up on the low side.

To Straighten the Needle.-Having levelled the compass, note if its two ends continue to point to exactly opposite degrees, while the compass is revolved completely. If they do, the needle is straight, and the pivot is in the centre of the graduated circle; if not, one or both of these are wrong. Level up the compass, then turn it until some graduation (say 90°) comes exactly to the north end of the needle. If the south end does not then point exactly to the opposite 270° division, lift off the needle, and bend the pivot pin to make it do so, noticing that every time the point is bent the compass must be turned slightly so as to put the north end of the needle at its 90° mark. Then turn the compass through 180°, or until the 270° mark comes exactly to the north end of the needle. Make a mark where the south end of the needle now is. Then remove the needle and bend it until its south end points midway between the 90° and the mark, while its north end is kept at 270° by moving the compass slightly.

Adjustment of the Pivot Pin.-After having straightened the needle, revolve the compass until a place is found where the two ends of the needle coincide with opposite degrees. Then rotate the compass through 90°. If the needle then coincides with opposite degrees the pivot pin is correct; but if not, the pivot pin is to be bent until it does. Repeat the operation until the needle coincides with opposite degrees while the compass is turned through a whole circle.

Pocket Magnifier.-In using a pocket magnifier for the compass readings, take care that it is held with its centre precisely over the point to be read and parallel to the graduated circle. Otherwise errors of several minutes may be made in a single reading.

Magnetic Attraction about the Person.-No part of the magnifier should be made of iron, as this will attract the

needle. Care should be taken that nothing is carried about the person, as knives, keys, &c., that may attract the needle. Trouble is sometimes experienced from felt hats, which have sometimes an iron wire round the inside of the brim to stiffen them.

Sight Slits. The sight slits are fixed by the instrument maker in line with the 360° and 180° divisions of the graduated circle. They may be tested by passing a fine thread through them, and observing whether it stands exactly over the 360° and 180° marks.

Remagnetising the Needle.-The needle sometimes loses part of its magnetism and becomes sluggish. It may be magnetised by drawing the north pole of a magnet several times from the centre to the south end of the needle, and the south pole in the same way from the centre to the north end of the needle, rubbing the magnet gently upon the needle. Take the magnet away from the needle while bringing it back to the centre. The needle should be held flat on a smooth hard surface while being magnetised. Bad action of the needle is more often due to defect in the point of the pivot pin. Remagnetising throws the needle. off the level, and this is to be adjusted by moving the sliding wire.

Adjustment of the Level.

1. To make the Collimation Line coincide with the Axis of the Telescope.-Drive in three pegs as at A, B, C, Fig. 163, an equal distance apart, say 3 or 4 chains. Set up the level at a exactly half way between A and B, and read the staff held on the pegs at A and B. Then set up at b midway between B and c, and read the staff held on the pegs B and C. Then whether the level is in adjustment or not, because it has been set up midway between A and B, any errors of adjustment will affect the staff readings at A and B equally and similarly; the difference of the readings will therefore give the correct difference of level of A and B. Similarly the difference of the staff readings at B and C will give the correct difference of level of B and c. Now set up at c, Fig. 164, as near to A as it is possible to read the staff held at a, and read the staff at A, B, C. From these staff readings compute the differences of level of A, B, C. Compare these with the correct differences of level previously found and

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compute the errors.

Then if the cross hairs are in the axis of the telescope, the error at c will be double the error at B; and if not, the diaphragm carrying the cross hairs must be moved by means of the screws at c, c, Fig. 104, until the error at c is double the

The following table shows the direction in which the cross hairs are to be moved :

2. To make the Spirit Level parallel to the Collimation Line, or in other words, to ensure that the collimation line is horizontal when the bubble of the level is at the centre of its run.

This is the most important adjustment of the level, as nearly the whole accuracy of the instrument in practice depends on it. The first adjustment need seldom be made, and when making it

care should be taken that the diaphragm screws are screwed up perfectly tight, and it will then seldom require to be interfered with again.

Drive in two pegs at A and B, Fig. 165, or select two good steady marks at these points, and set up the level at c exactly half way between A and B. Then whether the level be in adjustment or not, the staff readings at A and B will be equally and similarly affected, and the difference of these readings will be the correct difference of level of A and B. Now set up the level at d, Fig. 166, as close to a as it is possible to read the staff, and read the staff held on A. Now knowing the correct difference of level between A and B, compute from the last staff reading at a what

the staff reading at в should be. If now the actual staff reading at B does not agree with this, raise or depress the whole instrument by means of the levelling screws s, s, Fig. 104, until the reading at B is correct. Now bring the bubble to the centre of its run by means of the adjusting capstan screws at e connecting the level with the telescope. See now if the reading of the staff at A is altered, and if not, the adjustment is correct. If the reading at A is altered, the correct reading at в must be again computed from it, and the instrument made to read this by again depressing or raising it by the levelling screws s, s; the bubble being then again brought to the centre of its run by the screws at e, Fig. 104. Unless the level is very much out of adjustment, the alteration of