in the Field-Book by the number of links in the lines where they start and close, or cross.

The system of lines, having for stations the points c, d, e, f, g, 4, are minor lines, for the purpose of taking the crooked part of the north-eastern boundary of Lynch Wood. Other minor lines, running only from one secondary line to another, are of course less complicated.

The lines to distant objects, as Lynch church, &c., are only measured as far as the railway survey requires, the positions and distances of these objects being determined by intersection, as before observed.

These surveys are often made with the chain only, as it will be seen by referring to the figure, that the positions of the several base and secondary lines can be correctly determined in almost all cases without taking angles: however, taking the angles is a useful check on the accuracy of the work; and, more especially, where apprentices and other unskilful persons are employed to make these important surveys.

THE CIRCUMFERENTOR.

The circumferentor is sometimes used in surveys where great accuracy is not required: it consists of a flat bar of brass, B B, about 15 inches long, with sights, C C, at its opposite ends, and two narrow slits, bc, for observations: in

the middle of the bar is a circular brass box A, containing a magnetic needle, which as usual is covered with glass. The ends of the needle play. over a brass circle g, which is divided into 360°, in such a manner that the two 90° are at right angles to the line drawn through the sights. This instrument is usually supported on a tripod E, and when firmly fixed in the ground, it can be turned in any direction by means of its socket joint. When the mag

netic needle is properly ba

lanced, and moves freely in its horizontal position, the sights can

be turned towards the object, the direction of which is required, and the needle will retain its position of magnetic north; consequently the number of degrees which the angle contains, after moving from one object to another, can be read off. The great length of the magnetic needle increases the accuracy of the circumferentor, for, if it were small, and consequently the graduations on the brass circle proportionately small, the angle could not be read off with sufficient accuracy.

This instrument, as above described, is chiefly used in surveying mines, coal-pits, woods, &c.

The circumferentor is sometimes provided with a spirit-level, F, with its adjusting screws, a a, &c., a tangent screw m, a venier, &c., in which case it may be made to answer most of the purposes of a theodolite.

The needle should not be suffered to play longer than necessary, but be lifted off its centre, otherwise the delicate point on which it turns would soon be destroyed. This instrument usually has the east and west marked contrary to their true positions, in order that by the reading off of the needle, the actual direction of the line is shown.

To find the Bearing of an Object by the Circumferentor. Place the circumferentor over the station, turn N. sight to the object, and looking through the S. sight, adjust with the tangentscrew till the hair in the N. sight exactly cuts the objectthis part of the operation being the same as in the theodolite. When the needle has perfectly settled, read off the degrees to which its N. end points, from the N. or S. line of the compass-box, accordingly as the N. or S. end of the needle is in the N. or S. part of the compass-box; the angle thus read off is called the bearing of the object.

If the needle stand between two degrees, as between 40° and 41°, turn the instrument gently till it stand exactly at 40°, and clamp it; detach the sights, and bring them with the venier, with which they are connected, carefully back to the object: the number of minutes to be added to the angle will be shown by the first coincidence of a division on the venier with one on the horizontal plate; if the coincidence take place at the 27 division the angle will be 40° 27′.

The method of taking an angle with this instrument, without using the needle, is the same as with the theodolite.

On plotting the Bearings taken by the Circumferentor, &c. In laying down the work of the circumferentor, in cases where the angles are taken from the magnetic meridian, which are usually called

the bearings, that meridian is first drawn on the plan, and from a point in it the bearing of the first line from it is laid off with the protractor; the length of the line is next laid off, and through its extremity another meridian is drawn parallel to the former; the second bearing, and the length of the second line, are then laid off in the same manner; and so on till the work be completed, and the variation of the compass being known, the true meridian may then be drawn on the plan, that the work may have its proper position on the finished plan.

The above method is the same as that of laying down a wood, or road survey, by the theodolite, except in laying off the angles.

When the angles are taken with the circumferentor without reference to the magnetic needle, the method of laying down the work is the same as in theodolite-surveying.

NOTE. In coal-mines, when a pit or shaft is required to be sunk from the surface to a given point in the works below, it is usual to take the several bearings and distances, with this instrument, from the bottom of an existing shaft along the passages in the mine, to the required point; and then to repeat the same operation on the surface of the ground, without plotting the work on paper; by which means complete accuracy is frequently obtained.

OR FIELD OPERATIONS PREPARATORY TO THE CONSTRUCTION OF RAILWAYS.

SECTION I.

LEVELLING.

LEVELLING is the art of representing the inequalities of the upper boundary of any section of the earth's surface, and of determining the relative heights of any number of points in that boundary, above or below a line equidistant, at every point, from the earth's centre. This line is termed a level line*, and is that which water assumes when at rest.

The operation of levelling may be performed by various instruments, depending on the action of gravity: the most simple of which is the plumb-line, on which the mason's level depends; but its construction involves practical difficulties, such as to induce its rejection, excepting for very limited operations.

The fluid or water-level consists principally of a glass-tube, open at both ends, bent like the letter U, and partly filled with water; the surfaces of which, in the two arms of the tube, will stand on the same level by the action of gravity; and a line of sight, taken over these two surfaces, is therefore a level line. This instrument has been much improved by a Mr. Brown and others, but, in all its modifications, it is found inconvenient in practice.

There are also the Reflecting and Cambrian levels, which are useful only for operations of small extent, such as military engineering, &c.

* What is here termed a level line, is, in reality, a great circle of the earth; a small arc of which, on account of the great magnitude of its radius, may be considered as a straight and level line.

(1.) The instrument used for levelling, is called a spirit-level, or technically, a level.

(2.) This instrument is only a portion of a theodolite brought to its utmost practical perfection; its chief parts are the telescope A A, with the spirit-level DD beneath it, and two parallel plates with four conjugate screws K, K, &c., between them; to the lower of these plates are fixed the legs that support the instrument; and on the upper one rests a pillar that supports a strong brass plate G G, in which is fixed the compass H. This plate supports the telescope by means of two pillars F, F, called Y's from their shape, and from which the instrument has its name. The telescope is provided with a screw C, to adjust the object-glass, and four other screws to adjust the cross wires within it. The level is adjusted by the screw E, having a joint at the other end. The instrument is clamped by the screw I, and adjusted by the screw L.

ADJUSTMENTS.

(3.) There are three adjustments of the level; firstly, that of the line of collimation of the Telescope; secondly, that of the axis of the level with the axis of the Telescope; thirdly, that to make the Telescope always at right angles to the vertical axis of the instrument.

The two first adjustments are the same as those for the theodolite, which see the last is here given.