the course and distance, as in navigation, if the water be smooth and without a current.

In sea shoals, it is convenient to note at each observation the depth of the water found by the lead, and the drift and setting of the current by the log and compass, while the boat is at anchor, which may be done with ease and expedition enough. For while the boat rides at an anchor, her stern points out the setting of the current, and the log and glass will measure its drift.

And these ought to be noted on the draught, which may be thus:

The currents may be shewn, by drawing a dart pointing out its setting, and its drift by the Roman capital letters, the depth of the water by the small figures, and rocks by little crosses, &c.

LEVELLING.

Plate XIII. fig. 2.

EVELLING is the art of ascertaining the perpendicular ascent or descent of one place (or more) above or below the horizontal level of another, for various intentions and of marking out courses for conveyance of water, &c.

The true level is a curve conforming to the surface of the earth; as ABG.

The apparent level is a tangent to that curve; as ADE.

The correction, or allowance for the earth's curvature, is the difference between the apparent level and the true, as BD. The quantity of this correction may be known by having, in the right-angled triangle CAB, the two legs, AC the semidiameter of the earth (=1267500 perches) and AD=the distance of the object, to find the hypothenuse CD, from which taking CB; (=CA) the remainder will be the correction BD; but it may be obtained more practically thus ;

Square the distance in

four-pole chains and divide by 800,
or in perches and divide by 12800,
or in miles and multiply by

EXAMPLE.

8,

Required the correction for 20 four-pole chains =80 perches

mile?

800)20×20=400(.5 12800)80×80=6400(.5

1.25,and .25 × 25 × 8= .5

that is .5, or inch the correction required.

But, to save the trouble of calculation, we insert the following table of corrections.

A Table of Corrections.
The distances in four-pole Chains.

Dist. Cor. Dist. Cor.

The first thing necessary in levelling, is the adjusting of the level, which may be performed several ways: The following is very easy and practical.

Choose some ground which is not above 4 or 5

feet out of the level, for the distance of 8 or 10 chains length, and suppose it be AB (fig. 3.) and find the middle between A and B, which suppose to be C; plant the instrument at C: direct the tube to a station-staff, held up at A, and elevate or depress the tube, till the bubble is exactly in the middle of the divisions; then by signals direct your assistant at A, to raise or depress the vane, sliding on the station-staff, till the horizontal hair in the glass cuts the middle of that vane: then see how many feet, inches, and parts, are cut by the upper part of the vane, which suppose to be 3 feet 4 inches and 6 tenths.

In like manner direct to the other staff, at B and suppose the upper edge of that vane to cut at the height of 6 feet 5 inches and two tenths, then will these two vanes be on a level.

From 6 feet 5.2 inches subtract 3 feet 4.6 inches and reserve the remainder 3 feet 0.6 inches.

Now, remove the instrument as close to the higher station-staff as you can; so that the middle of the telescope may almost touch it. . Then bring the telescope as near to a level as the judgment of the eye will direct.

Measure from the ground, the height of the top of the telescope; and also of the bottom, in feet, inches and parts; suppose them to be 4 feet, 10.5 inches, and 5 feet 0.3 inches; then half the sum of these heights 4 feet 11.4 inches is the height of the centre of the glass; and to this add half the breadth of the vane, which suppose to be 1 inch and 5 tenths, and to the sum 5 feet 0.9 inches, add the preceding remainder 3 feet 0.6 inches; then let the person at B move his vane, till the upper edge cut 8 feet 1.5 inches, the sum of the preceding numbers.

Now, so elevate or depress the hair or the bubble, till the hair cut the middle of the vane at B, and at the same time the bubble stands at the middle of the divisions; and then will the instrument be duly adjusted.

If you have a mind to be more accurate, repeat the operation; but when you place the instrument at C, turn the tube at right angles to the line AB, and there set it level; then proceed with a repetition of the work. Only observe to cross-level it in this adjustment and in all future uses whatsoever.

Or the level may be adjusted thus: As before, first plant the instrument in the middle between A and B (fig. 4.) and observe the heights on the station-staves, which suppose to be as above; and consequently their difference, as before, is 3 feet 0.6 inches. Now measure from C towards the highest ground A, some distance that comes almost to A; suppose 4 chains to D, and DB will be 9 chains, and DA one chain: Then plant the instrument at D, direct the telescope to A, and, setting the bubble to the middle of the division, direct your assistant to move the vane, till the hair cuts the middle of it; and note down the feet, inches, and parts cut by the upper edge of the vane; which suppose to be 3 feet 8.4 inches: To this add the difference 3 feet 0.6 inches, and the sum 6 feet 9 inches reserve.

Now direct the telescope to the staff at B, level it, and direct your assistant to move the vane, till the hair cuts the middle thereof; and then, if the upper edge of the vane cuts the foregoing sum 6 feet 9 inches, the hair and bubble are truly adjusted. But if not, say, As BD less AD, is to the difference between the numbers cut by the upper