In order to know the value of his timber year by year, a book is prepared expressly for the purpose, noting what trees are felled either for sale or repair, and the number planted.

It is remarked by the most eminent arborists that, for every tree fallen, three should be planted to make good for casualties in the young plants.

The following is the method of keeping the book for measuring a fall of timber:

The foregoing observations are intended only as an introduction; practice and custom will be the best master.

226

PART V.

DESCRIPTION AND USE OF INSTRUMENTS IN SURVEYING AND PLOTTING.

Fig. 1, Plate 36. The theodolite is a very superior instrument, and generally considered to be the best for taking vertical and horizontal angles.

The telescope A B is fitted with achromatic glasses,* in order to obtain the greatest magnifying power, near to which are two fine hairs or cobwebs, attached to a diaphragm (Fig. 3, Plate 36), as at A, at right angles to each other. The object-glass (over which is a slide-shade to prevent the sun or rain acting on the glass) is moved by the milled screw b, till the distant object and the cross wires appear distinct; underneath is a spirit level D attached to the telescope at one end by a joint, at the other end by a capstan-head screw for adjusting it to the line of collimation. The whole rests on two Y's, and is confined by two clips.

The vertical arc E is fixed to a long axis, sustained and movable on two supporters fixed firmly at right angles to the horizontal plate F; on the upper part of the arc is a bar to which the Y's are fixed. The arc is graduated on one side into

* Achromatic, in optics, without colours, a term first used by M. de la Laude, in his Astronomy, to denote telescopes of a new invention, contrived to remedy aberrations and colours.

Aberration, in astronomy, an apparent motion in the celestial bodies, occasioned by the progressive motion of light and the earth's annual motion in her orbit.

degrees and half-minutes, each way from 0 to 90°, and subdivided by a vernier, as at d; to the axis of the arc is attached a movable microscope e, to read the angle more distinctly. The other side of the arc is divided into the number of links to be deducted from each chain, according to the angle of inclination or depression to reduce the hypothenusal measure to the horizontal measure. (See Table, No. 12.)

On the other side of the axis, at the point f, is a clamp screw to fix the telescope when taking an angle, and a tangent screw, g, to give the telescope a gentle motion, and bring the cross wires minutely to the object observed.

The horizontal limb F consists of two plates; the upper plate contains two verniers opposite each other, as at h (some instruments have three verniers). Attached to this plate are two spirit levels, i, at right angles to each other, with capstan-head screws for adjustment; also a compass or magnetic needle, k, having the circumference inside the box divided into 360° and half-minutes, and when not in use a spring is attached to lift off the needle from the point. This plate revolves with the whole of the before-described parts of the instrument on the lower plate, and when required, clamped by the screw 7 to the lower plate, and as before, when taking an angle, the tangent screw m will give it a slow and regular motion to bring the cross wires to the objects forming the angle.

The lower plate is chamfered, and divided into 360° and halfminutes; n is a microscope for reading the angles more distinct, and revolves round the lower plate; I is a collar that is attached to the main axis, having a clamp screw, G, which fixes the lower plate, and the tangent screw, H, gives it a slow motion.

The parallel plates K K are held together by a long axis, with a ball fitted into a socket upon the lower plate, ground accurately to fit it, attached to a conical axis passing through the upper parallel plate.

The four screws L are for bringing the whole instrument to a perfect level, the vertical arc being first fixed at zero. Then, by

turning the screws contrary ways to each other, bring the head of the instrument to a correct position for taking the observations.

Underneath the lower parallel plate is a female screw, by which it is fixed to the tripod when required, and, when shut up, forms a round staff.

The instrument is neatly packed into a mahogany box, in which is generally fitted a plummet and line used to bring the centre of the theodolite directly over the point of the angle about to be measured; also a turnscrew and pin to alter the screws when any part of the instrument requires adjustment.

There is also an inverted lens of different focus.

The best theodolites have another telescope underneath the lower limb, which moves either vertically or horizontally, and has a clamp and tangent screw to fix it to the object, to detect any error in the observation taken by the other telescope.

Previous to commencing operations with the theodolite, examine well the state of it, and ascertain if it is correct in all its adjustments; if not, the following methods must be adopted:

The first adjustment is of the telescope for collimation* and parallax. Set the instrument up firm, move the eye and object glasses by the screw b, till you can see clearly the cross wires and the objects at a distance; then the adjustment for parallax will be perfect.

Then direct the telescope to some sharp object that can be distinctly seen, such as the angle of a building, &c.; fix the telescope, and observe whether the horizontal hair coincides with the object; then turn the telescope round on its axis, and observe if the hair is exactly at the same place; if not, correct half the difference by moving one of the screws, a, and tightening the other; then reverse the telescope on the Y's, and if the hair does not coincide with the object, repeat the foregoing operation till in both positions it perfectly coincides with the same object.

*The line of collimation is the line of vision cut by the intersecting point of the cross hairs in the telescope, answering to the visual line, by which we directly point at objects with plain sight.

When the precise situation of the horizontal hair is ascertained, adjust the vertical hair in the same manner, turning the telescope at right angles.

When the two hairs are thus adjusted, reverse the telescope on the Y's, and observe if the intersection of the hairs coincide exactly with the point of the object; the line of collimation will be in adjustment.

The next adjustment is the horizontal and the vertical limb. Set the instrument as nearly level as possible by the legs or tripod; fix the whole by the clamp G, and loosen the upper part of the limb F; fix the vertical arc at zero, turn it round, and bring the telescope over two of the parallel plate screws L; then raise one of the staff screws, and depress the opposite; turn the telescope half round, and repeat the same over the two other screws; continue this operation till the bubble in the level at D remains in the centre of the tube; take the telescope out of the Y's and reverse it end for end. If the bubble remains in the same position as before, it is in good adjustment; if not, that end to which the bubble runs is too high; then, by the capstan screw at the end of the tube regulate half the difference the bubble has moved from the centre, and the other half by turning the screws L between the parallel plates. Return the telescope to its former position on the Y's; if then not correct, the same operation must be repeated both ways until it does; the two levels ii must be adjusted at the same time by the capstan screws, and when this adjustment is made satisfactory, the vertical arc is at perfect right angles with the horizontal limb.

As a further proof of the accuracy of the instrument, clamp the upper horizontal limb and loosen the lower plate; then move the instrument slowly round, and if the bubble, D, still retains its position, the two axis on which it revolves are correct; but if the bubble does not retain its position, then some defect exists in the axis, which can only be remedied by the makers.

When all these adjustments are made perfect, the instrument may always be levelled by the staff screws.