into feet and inches. The chain is of iron wire, each link being the hundredth part of the whole chain, which is 4 rods, or 66 feet, or 792 inches, so that each link is 7.92 inches in length. Every ten links is, for convenience of counting, marked by a piece of brass, with as many. tongues as the brass piece is tens of links from the extremity of the chain. A line is measured on the ground, as follows: Two persons take hold, one of each extremity of the chain, and one going in front, towards the point whose distance is to be measured, carries in his hand a staff and ten marking pins, of iron wire, each about two feet in length, sharpened to stick in the ground at the extremity of the chain measured off.

The one behind, by a motion of the hand to the right or left, indicates to the other whether the staff which is held at the end of the stretched chain is on the alignment of the distant point or not. As soon as he discovers it to be so, he makes a motion with his hand downward, and the other places a marking pin. Both then move on, the one behind taking up the marking pins which the other has left in the ground. When all the pins have been passed from one to the other, ten chains have been measured off.

For fixing the position of points of ground upon a map, the best instrument, when the survey is of moderate extent, is one which surveys and plots at the same time, called the

PLANE TABLE.

This consists of a rectangular board, mounted upon a three-legged stand, called a tripod, to which it is attached by a ball and socket movement, that is to say, there is a socket fastened to the tripod and a ball clasped by the socket, which moves in it, the ball being fastened to the underside of the table. This permits the table to be placed exactly horizontal. To ascertain whether it is so or not, a detached spirit level is placed upon the table temporarily, and the table is levelled by means of three screws, which pass through a horizontal circle of wood or brass which projects round the top of the tripod, the screws working against the table underneath. These screws are placed near the outer edge of the circle, and at distances of 120° from each other. If one of them be screwed in one direction, it lifts the table on that side; if in the opposite direction, it lets it down. In order to level the table by means of these screws the spirit level is placed over the line joining two of them, and by moving them the bubble is brought to the centre; this renders one line of the table horizontal. The spirit level is then placed in a direction perpendicular to its former one, and the bubble brought to the centre by

turning the third screw, leaving the others untouched; two lines of the table are then horizontal, and consequently the table itself. The spirit level, which is a tube of glass inclosed in one of brass, and containing spirits of wine, rests on short feet at the ends, one of which is made movable by a screw, and should be adjusted by reversing the level, end for end, on the table, after the bubble is first brought to the centre, when, if it departs from the centre, it must be brought back half by the foot screw of the level itself, and half by the levelling screws of the table. This process is to be repeated till in both positions of the level the bubble remains in the centre. A necessary appendage to the plane table is a brass ruler, with a thin edge, upon which are mounted either plain or telescopic sights, the line of vision being parallel to the edge of the ruler. Plain sights consist of two upright flat pieces of brass, one at each end of the ruler, facing each other with a narrow vertical aperture in each, to look through. Sometimes the aperture is made wider in the one towards the object, and a vertical thread or hair stretched down the middle of it, which serves for a sight. Larger orifices are made at some parts, in which to catch sight of the object, which is then brought down to a fine sight in the narrow part of the aperture.

When the sights are telescopic, the telescope may be mounted like that of a transit instrument (if the ruler is very wide) upon the upright piers of brass, by means of a small horizontal axis. Or upon a narrow ruler the telescope is supported at the top of a single column of brass, by a stout axis projecting from one side. At the focus of the object glass of the telescope two lines of spiders web cross each other exactly in the optical axis of the telescope. When the ruler is placed upon the table, by turning the table round a vertical axis called the axis of the instrument, the line of sights may be turned in any horizontal direction at pleasure. The telescope has a small vertical play also upon the horizontal axis, for the purpose of directing it to objects somewhat elevated or depressed. To use the instrument, place it over one end of the base line on the ground, so that a line to represent the base line drawn upon paper, stretched tightly and immovably upon the table, may be in the same direction. This is done by placing the edge of the ruler upon the line on the paper, and then turning the table upon the vertical axis of the instrument until a staff placed at the other end of the base line is seen upon the line of sights. There is a convenient arrangement for accomplishing this, consisting of what are termed a clamp and tangent screw, or screw of slow

This operation has generally to be repeated, as the levelling of the second line deranges a little the level of the first.

motion, an arrangement which is applied to almost all instruments. This consists of a screw working near its head, in a collar in which it has no longitudinal motion, the thread of the screw working in a piece of brass which is at pleasure loose from or screwed against the stand below, by a second screw, called the clamp screw, the first being called the tangent screw. When the instrument is "clamped," the tangent screw being turned, pushes the side of the table to which its collar is attached slowly away from the part of the tripod below, thus giving a slight motion to the table on its vertical axis. The line of sights being thus arranged in the direction of the base line on the ground, whilst the edge of the ruler coincides with the base line upon the paper, keeping now the edge of the ruler passing through one extremity of the latter, whilst the table is held immovable by the clamp screw, upon the tripod, turn the line of sights accurately towards one of the points to be plotted, and draw a line along the edge of the ruler upon the paper, marking it 1. Turn the line of sights successively to all the points in view to be plotted, drawing lines on the paper from the extremity of the base line on the paper, in the directions of these points, as before, and numbering them 2, 3, and so on, in order. Let the instrument now be taken up and carried to the other extremity of the base line, levelled, and the edge of the ruler being placed upon the base line on the paper, in a reversed position, bring the line of sights in the direction of the station at the first extremity of the base, by turning the table on its vertical axis, using the clamp and tangent screw, as before. Proceed in the manner just described, to draw lines also from this extremity of the base on paper, in the directions of the same objects, by sighting towards them in the same order, and numbering the lines as before. Where 1 meets 1 will be the position of the first object on the map; where 2 meets 2 the position of the second object, and so on.

The telescope of the plane table has sometimes an arrangement by which distances can be measured without the use of the tape or chain. Two sets of cross wires, as the spider lines are technically called, are placed at the focus of the object glass, the points of intersection of each pair being a small distance apart, the one above the other. If a staff be placed 100 feet from the telescope, and a space of exactly one foot be seen intercepted between the intersection of the wires; if then the staff be removed to a distance of 200 feet, two feet. will be seen intercepted on the staff, because, according to an optical law, the size of the image formed at the focus of the object glass is inversely as the distance of the object from the instrument, so that the space between the wires at the focus, where the image is formed, being occupied by 1 foot at the distance of

100 feet, the image of this 1 foot at the distance of 200 feet will occupy only half the space, or it will take an image of two feet to occupy the whole space.

To ascertain the proportions of the instrument by experiment, place a staff at such a distance that 1 foot may be intercepted between the intersections of the wires; measure this distance, and it will be the distance to be multiplied by the number of feet and fractions of a foot seen intercepted when the instrument is used for measuring distances. The staff used should be about two inches broad by one in thickness, and painted white, the divisions and numbers upon it being black or deep red, and made very distinct.

THE SURVEYOR'S COMPASS.

This instrument is a circular box of brass about six inches in diameter, and half an inch deep, mounted upon a tripod with ball and socket motion. The bottom of the box on the interior is silvered, and the circumference of this silvered bottom is graduated. In the centre of the bottom stands up a pivot upon which a long magnetic needle is accurately balanced. The top of the box is of glass, in order that the whole interior may be seen. Upon the box above, in the direction of a diameter, is a line of sights which may be plain or telescopic. The graduation is numbered from each end of the diameter, and runs to 90° each way.*

To survey a polygonal field with this instrument place it at one of the corners of the field, and direct the line of sights to the next corner along one of the straight boundaries of the field, and measure with a chain the length of this boundary line. Enter in a field book, ruled in three columns, in the first column the number of the station beginning with station 1; in the second column the Bearing (which would be, for instance, N. 30° E., if the line of sight were directed to the right of the north end of the needle marked with a cross, and the needle pointed to 30° on the graduated circle ;f) and in a third column the distance measured with the tape or chain. Take the instrument now to the corner whose position has just been determined, calling it station number 2, and determine the position of station number 3, a third corner of the field and the bounding line connecting stations 2 and 3 in the same manner, and so proceed quite ound the field. (See p. 233.) To plot this assume the point on the paper at

Sometimes the graduation is numbered from 0° to 360°, the 0 and 180 diameter being the line of sights.

+ If the graduation of the compass box be from 0 to 360, it will only be necessary to record the number to which the north end of the needle points.

which you will have the first corner of the field, draw through it a line to represent a magnetic meridian, or north and south line, and another line making an angle with this, equal to the first bearing, as 30° above, which will be laid off with the semi-circular protractor to the right of the meridian line if the bearing be east, and vice versâ; then from a scale of equal parts lay down the distance taken from the third column of the field book, and this will determine the second station of the map; through this draw a north and south line parallel to the first drawn, and lay off the second boundary by its bearing and distance, in the same manner, and so proceed till the plot is complete. The accuracy of the work will be tested by the last bearing and distance, reaching exactly to the first station from which the work commenced.

The same method may be pursued with the plane table, in an obvious manner. It is only necessary, as the table is moved from corner to corner of the field, to place the line on paper which has just been determined parallel to the corresponding line on the ground, by placing the ruler upon it, and sighting back from the 2d to the 1st station, turning the table on its vertical axis, for the purpose. The line joining the 2d and 3d station may then be drawn by sighting to the 3d station, the edge of the ruler passing through the 2d station in the paper, and by a scale of equal parts the length of this line laid down, and so on.

The compass may be employed to survey an irregular line, as a road, the border of a stream, a wood, a coast, &c., by taking stations sufficiently numerous to include portions nearly straight between them.

Thus:

Another mode is to run a straight line along the irregular boundary, and measure offsets, that is, perpendiculars to the main line, extending to the boundary at points where there are remarkable changes.

Thus:

The perpendiculars not only are measured with the chain, but the dis