Then

of the level DC stands at the middle of the tube. open the loops, and reverse the telescope. If the bubble still stands at the middle of the tube, the axis of the level is horizontal; but if not, it is inclined, the bubble being at the elevated end. In such case, raise the depressed, or depress the elevated end, by means of the small screw h, half the inclination; and then with the screws, at Mand R, bring the level to a horizontal position. Reverse the telescope in the Y's, and make similar corrections again; and proceed thus, until the bubble stands in the middle of the tube, in both positions of the telescope; the axis of the level is then horizontal.

Let the telescope be now revolved in the Y's. If the bubble continues in the middle of the tube, the axis of the level is not only horizontal, but also parallel to the line of collimation. If, however, the bubble recedes from the centre, the axis of the level is inclined to the line of collimation, and must be made parallel to it, by means of two small screws, which work horizontally; one of these screws is seen at q. By loosening one of them, and tightening the other, the level is soon brought parallel to the line of collimation; and then, if the telescope be revolved in the Y's, the bubble will continue at the middle of the point of the tube. It is, however, difficult to make the first part of this adjustment, while the axis of the level is considerably inclined to the line of collimation: for, allowing the level to be truly horizontal in one position of the telescope, after it is reversed, there will be but one corresponding position in which the bubble will stand at the middle of the tube. This suggests the necessity of making the first part of the adjustment with tolerable accuracy; then, having made the second with care, re-examine the first, and proceed thus till the adjustment is completed.

THIRD ADJUSTMENT. To make the level CD and the line of collimation perpendicular to the axis of the instrument, or parallel to the horizontal bar EE.

Loosen the clamp-screw S, and turn the bar EE, until

screws. By means of these screws, make the level CD truly horizontal. Then, turn the level quite round; if, during the revolution, it continue horizontal, it must be at right angles to the axis of the instrument about which it has been revolved. But if, after 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 instru ment 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.

THE WATER LEVEL.

8. The Water Level is an instrument that possesses the advantage of never requiring adjustment, and also of being very cheap; in fact, any ordinary workman may construct one. Having no telescope, it is impossible to take long sights, but for such work as is required to be done by the ordinary surveyor, it gives very good results.

Two brass cups, C and D, about one inch in diameter, and from four to five inches in height, are permanently attached to a hollow brass tube of three feet long and half an inch in di

ameter. The cups are for the purpose of receiving the ends E and F of

two bottles, the

bottoms of which have been cut off. cut off by means of a hot iron, or file.

D

B

The bottoms may be

The ends are fixed

The projecting axis g works in a hollow cylinder h, which forms the top of a stand. The tube, when the level is required for use, is filled with water (colored with lake or indigo), till it nearly reaches the necks of the bottles. After placing the stand tolerably level by the eye, withdraw both corks, and the surface of the water in the bottles will indicate a horizontal line in whatever direction the tube is turned. This level is well adapted to tracing contour lines as described in the next section.

LEVELLING STAVES.

9. The levelling staves 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.

The levelling staff is a necessary accompaniment to either of the levels described. Several kinds are used.

One of the best, consists of a staff 12 or 15 feet long, and graduated to feet, tenths, and hundredths. A sliding vane is made to move up or down by a cord and pulleys, and on the vane is a vernier, by means of of the staff may be sandths of a foot.

which the reading
effected to thou-
AB represents a

Fis D

portion of the staff, DC the moveable
vane, with an opening EF, through which
the graduation on the staff is seen.
the vernier of the vane, the 0 being de-
termined by the transverse line DC. To
render this line more distinct, the vane
is divided into four quarters, and the
alternate ones are painted black, which,
by their contrast with the white quar-
ters, show the line DC distinctly.

B

E

F

A

10. Another variety of levelling staff is shown in Pl. 4, Fig. 3. It is formed of two pieces, each about six feet long, one of which slides in a groove of the other, and bears a vane similar to that already described. It is grad

sight of the telescope is always directed to the centre of the vane. When the line of sight is less than six feet from the ground, the staff is reversed,-the vane run up the staff, and the readings made by means of the reversed figures at the right, where they are cut by the lower line of the vane. When the line of sight is more than six feet from the ground, the staff stands as in the figure, the reading is then made at the line bc, and the figures indicating the height, are found on the sliding part which carries the vane. The reading of the staff, as it now stands, is seven feet.

11. Another rod is sometimes used on which the figures are marked so plainly, that they may be read by the observer himself, without the aid of a vane; thus avoiding errors through ignorance or negligence of the rodman.

If the telescope used, inverts the object, the figures should be made inverted on the staff, so as to appear erect. Each of the rods described, has its advantages, and either one may be used according to the circumstances of the survey.

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, as G (Fig. 4). At equal distances of about 100 yards, on either side, and in the same line with the level, place the levelling staves, CE, BF. Make the level horizontal with the levelling screws. Then, turn it towards either staff, 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 Turn the level half round, and do the same in respect of the staff CE.

vane.

Let the telescope be now reversed in the Y's. Sight again to the staff BF, and note the exact height of the vane. Let the telescope be now turned half round, and the same be done for the staff CE. If the two heights last observed, are equal to those first noted, each to each,

instrument, and if the bubble has, at the same time, preserved its place at the middle point of the tube, the instru ment 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 CE, 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 col limation falls on the horizontal line gf.

LEVELLING IN THE FIELD.

13. The operation of levelling may be undertaken:

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

2d. For the purpose of obtaining a section or profile along a given line, as in the reconnoissance for a line of railroad.

3d. For the purpose of determining the contour lines in a topographical survey, as described in the next section.

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 station at which the work is begun, are called, for the sake of distinction merely, back-sights; and levels taken in the direction of the other station, foresights.

Before going on the field with the leve! rule three columns, as below, and head them, stations, back-sights,