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 1.05 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 in the middle of the divisions; and then will the instrument be duly adjusted.

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

Ag. 129.

D

D

B

Or the level may be adjusted thus: as before, first plant the instrument in the middle between A and B (fig. 129.) and observe the heights on the stationstaves, 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 midde 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 edge of the vane, and the number 6 feet 9 inches, so is the distance AD to a number, which added to that cut by the vane, when less than 6 feet 9, and subtracted from the number cut by the vane, when it is greater than 6 feet 9, will give a number to which let the assistant fix the vane; then so elevate or depress the hair or the bubble, till the hair cuts the middle of the vane at B, and the bubble stands in the middle of the divisions; for then the level will be adjusted. The operation may be again repeated, and at every station cross-levelled, which will confirm the former adjustment.

Or it will be still better to set the station-staves equally distant from the instrument (suppose about 16 or 20 perches each) at an angle of about 60° or so as to form nearly an equilateral triangle therewith, and level the 2 vanes (A and B fig. 130.) as before, which will be then both in the same horizontal level, whether the instrument be right adjusted or not, because one will be as much above or below the true level of the instrument, as the other, being at the same distance from it; then remove the instrument as near as may be to one of them, suppose A, and raise or lower the vane A to the exact level of the visual ray in the instrument, noting precisely how much it is moved, and have the other vane B moved just as much in order to bring them again to a level, allowing for the correction of the apparent level if it be a sensible quantity, then adjust the instrument to the level of the vane at B.

To adjust the rafter level (fig. 131.) which may be 10, 12 or 14 feet in the span AB; set it on a plank or hard

ground nearly level, and mark where the plumb lines cuts the beam mn, suppose at c, then invert the position by setting the foot A in the place of B, and B in that of A, marking where the line now cuts as at e; the middle point between c and e will be the true levelling mark.

To continue a level course with this instrument, set the foot A to the starting place, and move Bupward or downward toward D or E, till the point B be determined and marked for a level with A, then carry the instrument forward in the direction of C till the foot A rests at B, whence the point C is levelled as before, &c. Sights may be placed at r and s and the instrument adjusted to them, as before, by reversing them in the direction of some distant object.

After the instrument is duly adjusted, you may proceed to use it. Let the example be this annexed (fig. 132) where A, every where represents the level, and B the station-staves; and suppose the rout be made from a to e; first plant the instrument between the staves a and b at A direct the level to a B, bring the bubble to the middle of the divisions, and instruct your assistant so to place the vane, that the hair in the telescope cuts the middle of the vane; then in a book divided into two columns, the one entitled Back Sights, the other Fore Sights, enter the feet inches, and parts cut by the upper edge of the vane at a B, in the column entitled Back Sights.