The perfect adjustment of the instrument consists of the following particulars:

The horizontal circle must be truly level; the planes of the vertical circle must be truly perpendicular to the horizon; the line of sight or collimation must be exactly in the centre of the circle on which the telescope turns; the spirit level must be parallel to the line of collimation.

There are two methods in measuring an angle.

Theodolite. First: The angle measured from the meridian line to the chain line; second: The angle measured between two fixed objects.

To measure an angle from the meridian.

The theodolite being in perfect adjustment, place it firmly on the ground, and the parallel plates as nearly level as possible, and two staff screws in the direction of one of the chain lines. The centre of the instrument must be exactly over the station peg or angle point, which is proved by suspending a weight or plumb bob.

Fix the index on the vertical arc at zero, then proceed to level the instrument by the four staff screws. Turning them slowly contrary ways, raising one side and depressing the other, until the bubble is in the centre of the level under the telescope; then turn the instrument half round and repeat the same with the other two screws, continuing this operation till the bubble is stationary in the middle.

Then bring the zero of the horizontal limb F to the lower limb at 360°; its opposite will be 180°; and fix them by the clamp screw 7 on the upper plate (in all adjustments the tangent screw is used to move the instrument with greater minuteness).

Move the head of the instrument until the north end of the needle points exactly to 360° on the circle in the box, and fix it by the lower clamp screw G; loosen the clamp screw on the upper plate, turn the telescope until the cross wires cut the object set up on the chain lines, then read the angle on the limb F by the microscope n; as, for instance, 12° 40' N.E.; enter

the same in the field book, adjoining the line for which the angle was taken.

The same angle may be read by the needle if it is correct. Always repeat this operation, and if by the two readings there is a slight difference, take the mean.

When there is more than one instrument used in the survey, care should be taken to observe the difference existing between the needles of each instrument, so that the allowance may be made in plotting; they are frequently very much at variance. This applies to all other instruments having a compass.

When taking an angle, be careful to remove the chains and all other metallic attractions, otherwise the angle will be in

accurate.

To measure an angle between two lines.

The instrument must be fixed and adjusted in the manner before described; set the vernier of the horizontal limb to 360°, and clamp the instrument to the lower limb; bring the whole round until the cross wires of the telescope cut one of the objects set up for the required angle, and fix it by the lower clamp screw, G; then unscrew the clamp screw, l, on the upper plate, turn the telescope round until it cuts the other object, and read off the angle; always repeat the operation before moving the instrument.

This method of taking angles applies to the box-sextant, and all instruments not having a compass.

When more angles are required from the same point, they should be read off before moving the instrument.

To take a vertical angle.

The instrument being set firm in the ground, and put into perfect adjustment, raise or depress the telescope until the cross wires intersect the object; then fix the vertical arc by the clamp screw f, and bring the telescope minutely to the object by the tangent screw g; read off the angle by the microscope e. If the angle is below zero, it will be an angle of depression; if above zero, it will be an angle of elevation.

THE DIAPHRAGM.

Fig. 2, Plate 36. The diaphragm is a part of the telescope to the theodolite and spirit level. It is a circular ring of brass, made to act freely inside the telescope, and is most accurately divided into four parts (where only two cobwebs are used) at right angles to each other; from each of these points a cobweb is stretched across the ring, forming the vertical and horizontal wires, by which the angles and heights are determined. This ring is attached to the telescope by four screws, a, Fig. 1, the adjustment of which has been before described.

It not unfrequently happens to the surveyor to meet with an accident by breaking one or both the cobwebs, which at once destroys the use of the instrument. If unacquainted with the manner of replacing them, and has no other instrument, he is compelled to lose time, whilst it has to go to the makers for reparation; therefore it is well to know how to prevent these inconveniences by being able to repair them.

Find some place where there are cobwebs, wind them singly round a card, the same as winding silk, until you have plenty for the two.

Then take two narrow strips of card to act as weights, and with strong glue, or gum shellac, fasten the ends of the cobweb to each card sufficiently long to hang over the ring on each side; place the cobweb minutely over the two nicks cut on the ring, and drop a small quantity of the gum on the nick to fasten the cobweb; then cut off the two ends; serve the other the same, replace the diaphragm, and adjust it as described.

THE IMPROVED DUMPY LEVEL.

Fig. 3, Plate 38. The superiority of this level consists principally in its simplicity and compactness. By adopting an object glass of large aperture and short focal length to the telescope, considerable light and power is obtained. A is the telescope,

having a diaphragm, a, with cross wires placed in the usual manner; the internal tube or slide, which carries the eye-piece, B, &c., is nearly equal in length to the external tube or telescope, which, being sprung at its aperture, secures to the slide and its eye-piece a steady and parallel motion, when adjusting for distinct vision of a distant object by the milled screw C; D, the object glass, having a movable slide shade. The spirit level E is placed above the telescope, and attached to two rings passing round it; by the capstan-head screws, C, the air bubble of the level is adjusted for parallelism with the line of collimation.

The parallel plates and screws F are similar in every respect to those of the last-described instrument; d d represents the capstan screws for adjusting the telescope, and are protected by cases screwed to the horizontal bar G, which supports the telescope and spirit level; a compass-box is made to be attached to the horizontal bar when required.

The parallel plates are attached to the upper part of the instrument, and the whole is fixed to the tripod by a screw under the lower plate, as in the other instruments.

To examine and correct the collimation according to Mr. Gravatt's method.

On a tolerably level piece of ground drive in three stakes at intervals of about 4 or 5 chains, calling the first a, the second b, the third c.

Place the instrument half way between the stakes a and b, and read the staff A placed on the stake a, and also the staff B on the stake b; call the two readings A' and B'; then, although the instrument be out of adjustment, yet the points read off will be equidistant from the earth's centre, and consequently level. Now remove the instrument to a point half way between b and c, again read off the staff B, and read also a staff placed on the stake at C, which call staff C (the one before called A being removed into that situation); now, by adding the difference of the reading on B (with its proper sign) to the reading

on C, we get three points, A' B' C', equidistant from the earth's centre, or in the same true level.

Place the instrument at any short distance, say a chain beyond A, and, using the bubble merely to see that you do not disturb the instrument, read all three staves, or, more correctly, get a reading from each of the staffs a, b, c; call the three readings A", B", C". Now, if the stake b be half way between a and C, then ought C-C—(A”—A') be equal to B"-B'-(A’—A'); but if not, alter the screws of the diaphragm, and consequently the horizontal cobweb, or wire, until such be the case; and then the instrument will be adjusted for collimation.

To adjust the spirit bubble. Without removing the instrument, read the staff A, and say it reads A"; then, adding (A"-A' with its proper sign) to B', we get a value, say B". Adjust the instrument by means of the parallel plate screws to read B"" on the staff B.

Now, by the screws attached to the bubble tube, bring the bubble into the centre of its run. The instrument will now be in complete practical adjustment for level, curvature, and horizontal refraction for any distance not exceeding 10 chains, the maximum error being only 10th part of a foot.

Example. The instrument being placed half way between two stakes, a b (one chain from each other), the staff on a or A' read 6.53, and the staff on b or B' read 3.34, placing the instrument half way between the stakes b c (three chains from each); the staff on bread 4.01, and the staff on c read 5.31.

Hence, taking the stake a as the datum, we have

The instrument being now placed at d (say 5 feet from a, but the closer the better), the staff on a or A' read 4.01, on b or B' 1.03, and on c or C 3.07. Now, had the instrument been in complete adjustment (under which term curvature and refraction are included), when the reading on staff a was 4.01, the