size of the plan will admit of it, and finish the whole plan before taking the paper off the board. off the board. At the bottom of the paper, make a scale of the required proportion, carefully dividing it into tenths and hundredths as the case may be, and let all long lines upon the plan be measured off this scale. The short ones, that is, lines of offsets, lines of distances less than tenths, may be taken off the ivory scale, from which the scale upon the paper was first obtained.

Having made the scale, lay down your base line very accurately, and draw it carefully in with lake, marking the various stations upon it and its total length. Then take, with the compasses, the various lengths of the sides of the several triangles, of which the survey is composed, and lay off the different points of intersection, testing rigorously as you proceed, the constructed, with the measured lengths of the respective check-lines.

Do all this before an offset is put in, unless the offset be afterwards used as the point of a more convenient base line for another triangle.

beginning with the longest side, place a straight-edge upon it, about half an inch from the edge, and leaning hard upon the straight edge; turn up the edge of the paper by running the back of a knife under the paper against the straight edge, and run the glue brush, taking care not to have the glue too thick or too thin, upon the board under the paper, then, still keeping the straight-edge down, run the back of the knife over the paper close against the straight-edge, which will make the paper adhere partially to the board, then, taking a clean piece of paper, put it over the drawing paper, and with the back of the knife rub the glue hard on, until the glued edge adheres closely to the board. Then serve the opposite side of the paper the same way, taking care, in placing the straight-edge, to press well the straight-edge outward, so that the paper shall be as tight as the wet will allow it, before you glue down the second side. If this is carefully seen to, when the four sides have been glued in succession, the paper when dry, will be as tight and as smooth as a drum,

When this part of the plotting is found correct, draw the lines in very plainly with lake.

In marking off the several distances on the base line use one of the long scales, and placing it close against the given line, prick off with a fine needle the proper distances, and round the points, as centres, draw a small circle. Do this also in determining the points of inclination of the sides of the triangle.

Having finished these subsidiary lines, as they may with propriety be termed, proceed to the laying off the offset points. The best method of doing this, is to place the long scale above referred to, close against the given line, having the zero points of each coinciding, and get an assistant to read off the several distances thereon, whence offsets are taken, first going through the right offsets, then the left. An offset scale is now necessary, which is a small scale of two inches, divided in the same way as the long one, but the zero points being either edge of the scale. This is placed against the long scale, and the lengths on the measured line are determined by the long line, while the distance from any point, or offset therefrom, is determined by the offset scale; this latter point is alone marked. Practice and care will ensure considerable rapidity, as well as accuracy, in doing this.

When the scale is 2 or 4 chains to the inch, any offset, less than 10 links, must be done by the hand.

With reference to the division of the scales-the scales used for horizontal delineations are generally 2, 3, or 4, chains to the inch, or 20, 30, or 40 chains.

The division of the common and vertical sections is generally 5 or 50 feet, 10 or 100 feet to the inch.

LEVELLING;

Part the Fourth.

ITS NATURE AND OBJECTS.

CHAP. I.

LEVELLING is the art of representing the inequalities of the earth's surface, and of determining the relative heights of any number of points above or below a line, equidistant, at every point, from the centre of the earth. This line is what is understood by the term-a level line; it is that line which is assumed by water when at rest.

The instrument, used for the purpose of levelling, is called a SPIRIT LEVEL.

The Spirit Level.

This instrument is merely one portion of almost every other instrument, carried out to its greatest practical perfection. The bubble, which in most instruments forms only a subordinate part in the construction, is in this the chief, the only object of the instrument being to obtain a practical tangent to the earth's surface, or to place the line of collimation of the telescope in a truly horizontal line.

This instrument consists, like the others, of its parallel

plates, with their two pairs of conjugate screws, of its telescope, and its spirit level beneath. The telescope also, as in the case of the theodolite, has its milled-head adjustingscrew for the object-glass; and the moveable eye piece for neutralizing the parallax. The cross wires, however, are not arranged the same way as in the theodolite; sometimes one horizontal and two perpendicular wires are used instead.

The spirit level is furnished with its capstan-headed screws, for making it parallel to the axis of the telescope, vertically and laterally.

But in this instrument there is one contrivance which the theodolite does not possess-that of raising or depressing one of the Y's, or supports, on which the telescope rests, so as to have the axis of the telescope always at right angles to the axis of the instrument.

Adjustments.

There are two principal adjustments necessary in this instrument: the first to make the level and the telescope parallel; the second to make the axis of the telescope always at right angles to the axis of the instrument; in other words, to secure the line of collimation being perfectly level in any portion of a complete revolution of the instrument. The first adjustment depends upon what instrument is being used, and varies in each. There are, for instance, the Y level, Troughton's Improved, the Dumpy, all good levels, but differently adjusted. The second is almost the same in every instrument. It is thus arranged:

Set the telescope over any pair of conjugate screws, and make the bubble level; turn the instrument, till the tele

scope be over the conjugate pair: level it in this position; then turn it back to the first pair, and correct for any error that may have arisen from the last levelling, and continue till the bubble be central over the two pairs of conjugate screws; then turn the instrument one half revolution round, and, if the bubble still remain in the centre, the instrument is in adjustment; if not, the error can only be occasioned by the axis of the bubble, or which is the same thing, the axis of the telescope not being truly perpendicular to the centering of the instrument.

To correct this error, in the case of the Y level, raise or depress the moveable (Y) support by the milled-headed screw beneath, until the bubble be brought half-way to its proper position, and correct for the other half by the parallel screws. By repeating the correction two or three times, the greatest accuracy will be obtained.

It is necessary to examine the adjustment every morning before starting, and it should be seen to at every observation, though it will scarcely require re-adjusting the same day, I should observe, that there is, or ought to be, a cap over this adjusting (Y) screw which should be carefully kept on.

Levelling Staves.

These are generally made twelve feet high, divided into feet, and again into tenths of a foot, and subdivided for facility of computation, into hundredths. The method of arranging this subdivision, constitutes the difference between the several staves in use. The feet are distinguished by large red figures-the tenths are in black.