ticles of steel in the metal, of which the compass box is made.

To examine the graduations; set the index division of the nonius to the beginning of each degree of the theodolite, and if the last division of the nonius always terminates precisely, at each application, with its respective degree, then the divisions are

accurate.

Cautions in the use of the instrument. 1. Spread the legs that support the theodolite rather wide, and thrust them firmly into the ground, that they may neither yield, nor give unequally during the observation. 2. Set the instrument horizontal. 3. Screw the ball firmly in its socket, that, in turning the index, the theodolite may not vary from the objects to which it is directed. 4. Where accuracy is required, the angles should always be taken twice over, oftener where great accuracy is material, and the mean of the observation must be taken for the true angle.

To measure an angle with the theodolite. Let AB, BC, fig. 19, plate 9, represent two station lines; place the theodolite over the angular point, and direct the fixed sights along one of the lines, till you see through the sights the mark A; at this screw the instrument fast; then turn the noveable. index, till through its sights you see the other mark C; then the degrees cut by the index upon the graduated limb, or ring of the instrument, shew the quantity of the angle.

The fixed sights are always to be directed to the last station, and those on the index to the next. If the beginning of the degrees are towards the surveyor, when the fixed sights are directed to an object, and the figured or N. point towards him in directing the index, then that end of the index towards the surveyor will point out the angle, and the south end of the needle the bearing; the application of the instrument to various cases that

may occur in surveying, will be evident from what we shall say on that subject in the courseof this work.

OF THE COMMON PLAIN TABLE, fig. 1, plate 17.

The tabular part of this instrument is usually made of two well seasoned boards, forming a parallelogram of about 15 inches long, and 12 inches broad; the size is occasionally varied to suit the intentions of the operator.

The aforesaid parallelogram is framed with a ledge on each side to support a box frame, which frame confines the paper on the table, and keeps it close thereto; the frame is therefore so contrived, that it may be taken off and put on at pleasure, either side upwards. Each side of the frame is graduated; one side is usually divided into scales of equal parts, for drawing lines parallel or perpendicular to the edges of the table, and also for more conveniently shifting the paper; the other face, or side of the frame, is divided into 360°, from a brass centre in the middle of the table, in order that angles may be measured as with a theodolite; on the same face of the frame, and on two of the edges, are graduated 180°; the centre of these degrees is exactly in the middle between the two ends, and about 4th part of the breadth from one of the sides.

A magnetic needle and compass box, covered with a glass and spring ring, slides in a dovetail on the under side of the table, and is fixed there by a finger screw; it serves to point out the direction, and be a check upon the sights.

There is also a brass index somewhat longer than the diagonal of the table, at each end of which a sight is fixed; the vertical hair, and the middle of the edge of the index, are in the same plain; this edge is chamfered, and is usually called the fiducial edge of

the index. Scales of different parts in an inch are usually laid down on one side of the index.

Under the table is a sprang to fit on the pin of the ball and socket, by which it is placed upon a three legged staff.

To place the paper on the table. Take a sheet of paper that will cover it, and wet it to make it expand, then spread it flat upon the table, pressing down the frame upon the edges to stretch it, and keep it in a fixed situation; when the paper is dry it will by contracting become smooth and flat.

To shift the paper on a plain table. When the paper on the table is full, and there is occasion for more, draw a line in any manner through the farthest point of the last station line, to which the work can be conveniently laid down; then take off the sheet of paper, and fix another on the table; draw a line upon it in a part most convenient for the rest of the work; then fold, or cut the old sheet of paper by the line drawn on it, apply the edge to the line on the new sheet, and, as they lie in that position, continue the last station line upon the new paper, placing upon it the rest of the measures, beginning where the old sheet left off, and so on from sheet to sheet.

To fasten all the sheets of paper together, and thus form one rough plan, join the aforesaid lines accurately together, in the same manner as when the lines were transferred from the old sheets to

the new one. But if the joining lines upon the

old and new sheets have not the same inclination to the side of the table, the needle will not point to the original degree when the table is rectified. If the needle therefore should respect the same degree of the compass, the easiest way of drawing the line in the same position is to draw them both parallel to the same sides of the table, by means of the scales of equal parts on the two sides.

To use the plain table. Fix it at a convenient part

of the ground, and make a point on the present that part of the ground.

Run a fine steel pin or needle through this point into the table, against which you must apply the fiducial edge of the index, moving it round till you perceive some remarkable object, or mark set up for that purpose. Then draw a line from the station point, along the fiducial edge of the index.

Now set the sights to another mark, or object, and draw that station line, and so proceed till you have obtained as many angular lines as are necessary from this station.

The next requisite, is the measure or distance from the station to as many objects as may be necessary by the chain, taking at the same time the offsets to the required corners or crooked parts of the hedges; setting off all the measures upon their respective lines upon the table.

Now remove the table to some other station, whose distance from the foregoing was previously measured;, then lay down the objects which appear from thence, and continue these operations till your work is finished, measuring such lines as are necessary, and determining as many as you can by intersecting lines of direction, drawn from different stations.

It seems to be the universal opinion of the best surveyors, that the plain table is not an instrument to be trusted to in large surveys, or on hilly situations; that it can only be used to advantage in planning the ground plot of buildings, gardens, or a few small parcels of land nearly on a level.

Mr. Gardner, whose authority as a surveyor is inferior to no one, asserts, that the plain table surveyors, when they find their work not to close right, do often close it wrong, not only to save time and labour, but the acknowledgement of an error; which they are not sure they can amend.

In uneven ground, where the table cannot in all

stations be set horizontal, or uniformly in any one place, it is impossible the work should be true in all parts.

The contraction and expansion of the paper according to the state of moisture in the air, is a source of many errors in plotting; for between a dewy morning and the heat of the sun at noon, there is a great difference, which may in some degree be allowed for in small work, but cannot be remedied in surveys of considerable extent.

OF THE IMPROVED PLAIN TABLE, fig. 2, plate 17.

To remedy some of the inconveniences, and correct some of the errors to which the common plain table is liable, that which we are now going to describe has been constructed. It is usually called Beighton's plain table, though differing in many respects from that described by him in the Philosophical Transactions.

It is a plain board, 16 inches square, with a frame of box or brass round the edge, for the purpose of being graduated. On the sides, A B, C D, are two grooves and holdfasts for confining firmly, or easily removing the paper; they are disengaged by turning the screws under the table from the right towards the left, and drawn down and made to press on the paper by turning the screws the contrary way. When the holdfasts are screwed down, their surface is even with that of the table. There are two pincers under the table, to hold that part of paper, which in some cases lies beyond the table, and prevent its flapping about with the wind.

the

The compass box is made to fit either side of the table, and is fixed by two screws, and does not, when fixed, project above one inch and an half from the side of the table.

There is an index with a semicircle, and telescopic