Level. cale truly level. In this facile mode of adjustment against it precisely while the telescope is turned round Levelling. confifts the new improvement of the instrument; and on its Ys, it proves, as before mentioned, that the axis or, in other words, the difference in their distance from the supporters is also sometimes fixed a compass- serve, that a plumb-line, hanging freely in the air, the tangent to it the apparent level. Hence it ap- Multiply the number of Gunter's decimal statute Leacb's lice The horizontal motion being thus adjusted, the rims chains that are contained in length between any two land naviff of the Ys are to be opened, the telescope taken off stations where the levels are to be taken by itself, and patien. and laid the contrary way upon the supporters. If the the product arising therefrom again by 124; which is bubble of air then rests exactly the same, the level and a common multiplier for all manner of distances for telescope are adjusted rightly to one another ; but if this purpose on-account of the earth's curvature: then the bubble does not remain the same, the end to which divide the second product arising therefrom by 100,000; the air bubble goes must be noticed, and the distance or, which is also the fame, with the dash of the per of it from the telescope altered; correcting one half cut off five figures on the right hand side of the prothe error by the screws cc, and the other half by the duct, and what remains on the left side is inches, and screws ee. the five figures cut off decimal parts of an inch.” B" TIC a 66 ner. Chains. Inches. Chains. Inches. Chains. Inches . 65 5.31 70.06 to Levelling. The following is A Table of Curvature of the Earth The operation may be begun in the following man. Levelling. and shows the quantity below the apparent level at Let the first station be at 1, equally distant from the two points A and B, which themselves are distant cond limit B; in the fourth, the height indicated at 40 2.00 the station-Itaff B, viz. 6. O. O. Lastly, in the fifth co ftaves are to be held ; observing that B, which was the second limit in the former operation, is the first in this. 80 8.00 C; in the fourth 5. 6. 2; in the fifth 560, the di- stance between B and C. It being impossible, on account of the inequality of II 0.15 24,0.72 37 1.711 90 10.12 the ground at the third station, to place the instrument 12 0.18 250.78 38,1.80' 95 11.28 in the middle between the two station-staves, find the 130.21 26'0.84 39 1.91 100 12.50 most convenient point as at 3; then measure exactly Levelling is either simple or compound. The for- how far this is from each ftation-staff, and you will mer is when the level points are determined from one find that from 3 to C is 160 yards; from 3 to D, 80 ftation, whether the level be fixed at one of the points yards; and the remainder of the operation will be as or between them. Compound levelling is nothing more in the preceding station. In the fourth operation, we must endeavour to com- the last. Mark out, therefore, 80 yards from the sta- 4 the height E; and this must be carefully attended to, as by fuch Feet. Inches. compensations the work may be much facilitated. Pro- 6 ceed in the same manner with the eight remaining staFrom B to D be 9 tions, observing to enter every thing in its proper co lumn; and when the whole is finished, add the sums of The difference 3 each column together, and then subtract the lesser from the greater ; the difference, which in the present case is line on, fig. 4. either above or below the plan ; which 7. . and delineate the ground with its inequalities. Pro ceed m a a > . a feet i. A 21 4 3D E 16 3 3591 OF F 10 OH 19 H 5 Levelling. ceed in the same manner from flation to station, till work ; by which it is easy to form a just estimation of Levelling, you arrive at the last N, and you will have the profile the land to be dug away, in order to form the canal, This method answers very well where only a gene- Fig: 6. gives an example of compound levelling, instrument. The following is a table of the fect. 10.;yards. 60 9 901 the following, till this part of the work is finished; С 3 40 there remains then only to delineate in detail the ground D 3 9 between the station-staves, the diftances in this example E 5 17 9 250 being assumed larger on account of the detail. 6G 5 03751 G 0 300 3 1000 95 o 106 9240; ! To set off the height of the brink of the river above but they are omitted to avoid confufion. In the 4 reduce the apparent level to the true one, which is alinches; then, laying off upon the line oz the distance ways necessary where the length is considerable. At the from the first to the last stake, let fall from thence a last limit we get the height from N to o; then from o to perpendiculas, and set off thereon 4. 10. O to a, which I; from I to K, fig. 7. &c.; all which added together, gives the height at the first stake ; or, which is the and then corrected for the curvature, gives 47 feet 3 same, the height from the edge of the river above the inches. Now, by adding each column together, and surface of the water, as is evident from the section. subtracting one from the other, we have 51 feet 9 Drive a second stake at 6, in a line between the limits; inches for the height which the point A is above the place the itation-staff upon this stake, and observe the bottom of the bafon, and which will cause the jet height 4. 6. interfected by the cross hairs, the instru- d'eau to rise about 45 feet. The general section of ment still remaining in the same situation. Set off on this operation is shown at fig. 7, 8. but an exact profile the level-line the distance from the first stake a to the of the mountain is more difficult, as requiring many second b; and then let fall a perpendicular, and mark operations ; though some of these might be obtained upon it 4.6 to b, which gives the height of the ground by measuring from the level line without moving the at this place. instrument. The small hollow c is marked out by driving down The latt example given by our author is likewise a third stake even with the ground, in the middle of it from M. le Febure, and includes a length of near five at c; but the exact distance of the second stake b from German miles (25 of ours) in a straight line, and 9 or the third must be marked upon the level line : then 10 (45 or 50 English) including the tụrnings and let fall a perpendicular from c, and set off upon it windings. In this the declivity of the river Faynox 6. 8. 9, pointed out by the cross hairs on the staff, was measured from Lignebruk to Villebourg. The which determines the depth of the hollow, as appears first operation was to drive stakes at several parts of from the figure. As the distances between the stakes the river even with the water's edge; the first of are now very short, they can eatily be marked by the which a little above the mills of Lignebruk showed operator, who can settle any little inequalities by a the upper water-mark, and another showed the lower comparison with those already ascertained. Proceed water-mark at the same mills. Two stakes above and thus with the other stations till you arrive at the laft, below the mills of Mazurance, somewhat more than and you will always obtain an accurate section of your half way between Lignebruk and Villebourg, pointed B 2 ους feet 10 : |