Imagens das páginas

In some cases the practice of employing a payclerk has been introduced to pay for all the road expenses, in order to relieve the surveyor from all trouble about pecuniary matters, and at the same time to remove as much as possible all temptation to swerve from his duty. This practice has been attended with the best effects, and cannot be too strongly recommended.

[ocr errors]



The principal instruments employed in surveying and laying out roads are theodolites, spirit levels, and sextants.


Theodolites in careful and experienced hands are the best instruments for laying out a road, and for taking horizontal angles and intersections. The rates of inclination can be determined at once by means of the vertical arch, without any measurement by the chain being required: they are decidedly the best instruments.

Theodolites are made of various sizes and prices; but those that are five inches in diameter, and cost about 17/., are the most suitable for road purposes.

These instruments are divided on the limb into spaces of thirty minutes, and by means of a vernier, single minutes can be read off with great precision.

They are furnished with a good telescope and spirit level, besides two levels on the limb set at right angles to each other, and a magnetic needle or compass in the centre, which is of use in getting the magnetic bearing of any line in the survey, or of taking the bearings independent of the divisions on the limb.

The theodolite is used in the following manner in surveying a road: —

When the line of direction is fixed upon, the theodolite is set up over the first point in the survey; it is then adjusted by means of the spirit levels, so as to be perfectly level. The eye piece of the telescope is moved in or out until the hairs are seen distinctly; and the object glass is adjusted to distinct vision, according to the distance of the levelling staff from the instrument. Zero on the limb is then brought to coincide with zero on the vernier plate, and the limb and plate are then clamped together. After this is done, the whole head is turned round until the north point on the compass box coincides with the north point of the needle. The limb is then screwed fast, and the vernier plate undamped and turned round until the staff is seen through the telescope: the vernier plate is then clamped, and the observation completed by turning the tangent screws of the limb and of the vertical arch, until the centre of the vane exactly corresponds with the centre of the cross hairs in the telescope. The degree and minute on the limb and vertical arches are then read off and entered in the field book.

The distance from the instrument to the staff is then measured by the chain, and all offsets are at the same time measured and entered in the book. The length of the distance line is then carefully entered, and the theodolite removed and again set up directly over the point previously occupied by the levelling staff: this may be done by means of the plumb line usually attached to the instniment.

The next operation is to adjust the instrument perfectly level, and to send the staff back to the point originally occupied by the theodolite. The vane having been previously adjusted to the exact height of the centre of the telescope, the head of the instrument is then turned round until the staff is seen in the field of view of the telescope: the head is then clamped, and the bisection made by means of the tangent screw; the vernier plate still remaining steadily clamped to the limb. The vertical arch is then examined, to see if the degree and minute correspond with those previously observed: if not, the first observation must be repeated. The vernier plate is then undamped, and the telescope turned round towards the next line of direction until the staff appears in the field of view; when this is effected, the vernier plate is clamped, and the observation completed as before. In this way the survey is carried on; and the perpendiculars and rates of inclination are afterwards calculated, and the plan and section laid down in the usual way.

Spirit Levels.

Troughton's levels, which are considered the best, are usually made with very powerful telescopes and delicate ground spirit levels. These instruments are usually fourteen inches long, but some are eighteen and others twenty inches long. They cost from 19,1. to 18/., and are so well balanced and secured that they will not require, with proper care, for a long time, any adjustment.

The method of using these instruments is as follows:]

When the direction of the road has been marked out, a line is measured by a chain commencing at the beginning of the new line, and terminating at that point where the inclination of the surface of the ground changes, or where the line of direction changes. This distance is carefully entered in the field book. The spirit level is set up as nearly as possible in the middle of this line, and a levelling staff with a vane is held by assistants at each extremity of the line: the telescope is then adjusted for distinct vision, and its axis brought to be truly vertical, by means of the spirit level and parallel plates. The telescope is then directed to the staff, which is placed at the commencement of the line, and the assistant is directed to lower or raise the vane until it is bisected by the cross hairs in the telescope: the height marked by the vane on the staff is then set down in the field book in the column headed (back observation). The telescope is then turned round until the staff at the termination of the line is perceived in the field of view; the necessary signals are then given to lower or raise the vane on the staff until its centre coincides with the cross hairs in the telescope: the height of the vane on the staff is then entered in the field book in the column marked for observation, and the magnetic bearing of the line is also observed and set down in another column. Some

« AnteriorContinuar »