the revolution, the level CD be not horizontal, rectify half the error with the screws at M and R, and half with the levelling screws. Then place the bar EE over the other two levelling screws, and make the same examinations and corrections as before; and proceed thus, until the level can be turned entirely around without displacing the bubble at the centre. When this can be done, it is obvious that the level DC and the line of collimation, are at right angles to the axis of the instrument, about which they revolve; and since the axis is carefully adjusted by the maker, at right angles to the bar EE, it follows, that the line of collimation, the level DC, and the bar EE, are parallel to each other.

The level is now adjusted. When used, however, it is best to re-examine it every day or two, as the work will be erroneous unless the instrument is accurately adjusted.

LEVELLING RODS.

9. The levelling rods are used to determine the points at which a given horizontal line intersects lines that are perpendicular to the surface of the earth, and to show the distances of such points of intersection from the ground.

There are two kinds of rods used by engineers, known as the Boston and New York rods.

They are both sliding rods, divided into feet, tenths, and hundredths of feet; and the readings, by means of a vernier, are made to thousandths of a foot.

10. The Boston Rod is formed by two pieces of hard wood, about six feet and a half in length, the one sliding through grooves, along the other, in both directions.

A vane or target, six-tenths of a foot in width, divided into four equal rectangles by a horizontal and vertical line, passing

through the centre, is permanently connected with one extremity of the sliding piece. The two diagonal rectangles of the target, are usually painted black or red, and the other two white, so that the centre and the horizontal dividing line, may be distinctly seen.

There are two verniers, one at either end of the second piece, by means of which the readings, indicating the height of the target, are read.

When the height is less than six feet, the reading is made by the vernier at the top of the rod; and when it is greater than six feet, the rod is reversed, which brings the other vernier to the top; the target is then run up to the required height, and the reading made as before.

New York Rod.

11. This rod, which is shown in the engraving, is cut in two parts, so that both ends may be exhibited. It is made of satin-wood, in two pieces, like the former, but sliding one from the other, always in the same direction, so that the same end is always held on the ground, and the graduations start from that point. In this rod, as in the other, a target is used to indicate where the horizontal line cuts the rod.

The face of the target is divided into quadrants, by a horizontal and a vertical diameter; and these diameters are the boundaries of the alternate colors with which the diagonal quadrants are painted.

The opening, in the face of the target, is a little more

chamfered, and divided into ten equal spaces, corresponding to nine hundredths on the rod: hence, the vernier reads to thousandths of a foot. For heights less than six feet, the target is moved along the sliding part, to which it is slightly attached by springs, and to which it may be permanently attached by a clamp-screw, and the reading is made by the vernier on the target. When the height exceeds six feet, the slide and target are run up to the requisite height, and the reading is made by the vernier at the top of the staff.

Tests of Adjustment.

12. There is a method of testing the adjustments of the Y level, which ought not to be neglected, since all the results depend on the accuracy of the instrument. The method is

this:

The level being adjusted, place it at any convenient point, us G (Pl. 4, Fig. 4). At equal distances of about 100 yards, on either side, and in the same line with the level, place the levelling rods, Cb, BF. Make the level horizontal with the levelling screws. Then, turn it toward either rod, as BF, and run the vane up or down, as required, until the intersection of the hairs strikes the centre: then make the slide fast, and note carefully the height of the vane. Turn the level half round, and do the same in respect of the staff Cb.

Let the telescope be now reversed in the Y's. Sight again to the rod BF, and note the exact height of the vane. Let the telescope be now turned half round, and the same be done for the rod Cb. If the two heights last observed are equal to those first noted, each to each, the line of collimation is perpendicular to the axis of the instrument; and if the bubble has, at the same time, preserved its place at the middle point of the tube, the instrument is truly adjusted.

For, had the line of collimation been inclined to the axis of the level, it would, in the first instance, have taken the direction AF or Ad; and when turned half round, it would have taken the direction AE or Ab. The telescope being reversed in the Y's, and again directed to the staff BF, the line of collimation would take the direction Ad or AF, and when turned to the staff Cb, it would take the direction Ab or AE: and the two distances BF, Bd, or Cb, CE, can only be equal to each other when the line of collimation falls on the horizontal line gf.

LEVELLING IN THE FIELD.

13. The operations of levelling may be undertaken :

1st. For the purpose of determining the difference of level between two given points:

2d. For the purpose of obtaining a section or profile along a given line, as in the preliminary surveys for railroads and canals:

3d. For the purpose of determining the contour lines in a topographical survey, as described in Section II.; and,

4thly. For the purpose of determining the volume of any given mass of earthwork or masonry; as the measurement of excavations and embankments for canals and railroads: and,

5thly. For the purpose of determining and indicating boundaries for filling and excavation; such as setting stope stakes, &c.

DIFFERENCE OF LEVEL BETWEEN TWO POINTS.

14. When it is proposed to find the difference of level of any two objects, or stations, all levels made in the direction of the

distinction merely, back-sights; and levels taken in the direction of the other station, fore-sights.

Before going on the field with the level, rule three columns, as below, and head them, stations, back-sights, fore-sights.

Find the difference of level between any two points,
as A and G (Pl. 4, Fig. 5).

The level being adjusted, place it at any point, as B, as nearly in the line joining A and G, as may be convenient. Place a levelling rod at A. Make the level horizontal by means of the levelling screws; turn the telescope to the rod at A, and direct the rodman to raise the target until the horizontal line ab pierces its centre; then note the distance Ab (equal to 10 feet in the present example) and enter it in the column of back-sights opposite station 1.

Send the rodman forward to some point, as N, in the proposed direction, and sight to the rod as before; enter the distance Na, equal to 3 feet, in the column of fore-sights opposite station 1, (B). Then remove the level to a convenient point, as C, (2). Direct the rodman to run up the vane to the proper height; then make the back-sight, and enter it, Nd = 11.6 feet,