Chapter I: Reconnoissance

26. The object of the reconnoitre is to find approximately the place for the road, (i. e. within half of a mile,) to find the general form of the country, and to choose that part which with reference to the expected traffic, shall give the best gradients; to determine the elevations of summits upon competing routes; and, in fine, to prepare the way for the survey.

GENERAL TOPOGRAPHY.

27. The general topography of a country may be ascertained by reference to State maps, where such exist, and when not, by riding over the district. The direction and size of watercourses, will show at once the position of summits.

28. Water flowing as in fig. 1, indicates a fall from B to E; and also traverse slopes from _a a_ and _c c_ to _d d_.

29. Fig. 2 shows a broken ridge _a a a_ from which the water flows in both directions; and in general, the sources of streams point towards the higher lands.

30. If it be required to join the points A and D by railroad, (fig. 3.) it may be better to pass at once from A through B and C, than to go by the streams F E, F′ E′. By the latter route the road would _ascend_ all of the way from A to E; and descend from E′ to D. By the first if it requires steep gradients to rise from A to B, and to fall from C to D, still if the section B C is a plateau, and if the rise between A and B and A and E is the same, by grouping the grades at B and C we may so adapt the motive power, as to take the same train from A to D without breaking. The general arrangement of grades by the line A B C D is then as fig. 4; and A F E E′ F′ D, as in fig. 5. The saving in this case is by length, as the same amount of power is required to overcome a given ascent.

31. Valleys generally rise much faster near their source, than at any point lower down; also the width increases as we approach the debouch. Fig. 6 shows the cross sections of a valley from its source to the mouth.

32. In the case of parallel valleys running in the same direction, the form will be as in fig 7. Let 1 2, 1 2, etc., represent a datum level, or a horizontal plane passing through the lowest point. The line _a b_, shows the height of the bottom at B; _c d_ that at D, _e f_ that at E, and _g h_ that at C. The broken lines _i, k, l, m, n_, show the general form of the land. Now by the route _m m m m_, from A to F, we have the profile _m m m m_, fig. 8, by _n n n n_, the profile _n n n n_, and by _o o o_, the profile _o o o_.

33. In the case of parallel valleys running in opposite directions, as in fig. 9, we have the form there shown; and the profiles corresponding to the several lines are shown in fig. 10. As we should always adopt the line giving the least rise and fall, other things being equal, it is plain which line on the plan we must follow.

34. In passing from A to B, figs. 11 and 12, by the several lines _c, d, e, f_, we have the profiles shown at _c, d, e, f_, from which it appears, that the nearer we cross to the heads of streams, the less is the difference of heights.

35. If we wish to go from A to B, fig. 12 (_a_), we should of course take first the straight line; but being obliged to avoid the hill C, on arriving at _d_, we should not try to recover that line at _e_, but proceed at once to B. Also as we are obliged to pass through _d_, we ought to go _directly_ to _d_ and not by the way of _c_; and the same idea is repeated between A and _d_; the last line being A _b d_ B. Few rules can be given in the choice of routes. Practice only will enable the engineer to find the best location for a railroad.

BAROMETRICAL LEVELLING.

36. The relative height of summits, the rate of fall of streams, and absolute elevation, within a few feet, may be easily, rapidly, and cheaply found by the barometer. This also affords an excellent check upon subsequent levelling operations. The results thus obtained depend upon the physical property, that the density of the air decreases as the square of the height.

37. The barometer is a glass tube, partly filled with mercury, having a vacuum in the upper part. By it the exact density of the air at any point is determined. Accompanying are two thermometers; one _attached_, showing the temperature of the _barometer_; the other _detached_, showing the _atmospheric_ temperature.

38. Knowing now the manner of finding the density of the air at any two points, and also the relation between density and height, the operation of levelling by the barometer is very simple.

The _modus operandi_ is as follows, (see tables A, B, C, and D):—

Let us have the notes.

Barom. Attached Therm. Detached Therm.
Upper Station, 29.75 28.5 27.9
Lower Station, 26.80 36.8 36.3
Latitude 46° N.

We have by table A, against the bar. point, 29.75, 6108.6
also by table A, against the bar. point, 26.80, 5276.6
——————
The difference 832.0
Diff. of attached therm. 36.8°- 28.5° = 8.3° (table B) -12.2
——————
819.8

Double the sum of detached thermometers multiplied by 1/1000 of
819.8 is
2(27.9 + 36.3) × .8198 = + 105.3
——————
925.1
Correction (see table C) for lat. 46° N. and approximate height
925.1 + 3.1
——————
928.2

Final correction by table D. The barometer at the lower station
being 26.80, and the tabular number against 27.56 being 0.22, that
for 26.80 will be 0.31, and we have

1000 to .31 as 928.2 to 0.287, or 0.3,

which add to 928.2 and we have as the final height

928.5 metres, or 928.5 × 3.28 = 3045.48 feet.

The tables above referred to, are those of Mr. Oltman, and are considered as the most convenient and reliable of any published.

TABLE A.

┌───────────────┬───────────────┐
│English Inches.│ Metres. │
├───────────────┼───────────────┤
│ 14.56│ 418.5│
│ 14.61│ 440.0│
│ 14.65│ 461.5│
│ 14.68│ 482.9│
│ 14.72│ 504.2│
│ 14.76│ 525.4│
│ 14.80│ 546.6│
│ 14.84│ 567.8│
│ 14.88│ 588.9│
│ 14.92│ 609.9│
│ 14.96│ 630.9│
│ 15.00│ 651.8│
│ 15.04│ 672.7│
│ 15.08│ 693.5│
│ 15.12│ 714.3│
│ 15.16│ 735.0│
│ 15.20│ 755.6│
│ 15.24│ 776.2│
│ 15.28│ 796.8│
│ 15.31│ 817.3│
│ 15.35│ 837.8│
│ 15.39│ 858.2│
│ 15.43│ 878.5│
│ 15.47│ 898.8│
│ 15.51│ 919.0│
│ 15.55│ 939.2│
│ 15.59│ 959.3│
│ 15.63│ 979.4│
│ 15.67│ 999.5│
│ 15.71│ 1019.5│
│ 15.75│ 1039.4│
│ 15.79│ 1059.3│
│ 15.83│ 1079.1│
│ 15.87│ 1098.9│
│ 15.91│ 1118.6│
│ 15.95│ 1138.3│
│ 15.98│ 1157.9│
│ 16.02│ 1177.5│
│ 16.06│ 1197.1│
│ 16.10│ 1216.6│
│ 16.14│ 1236.0│
│ 16.18│ 1255.4│
│ 16.22│ 1274.8│
│ 16.26│ 1294.1│
│ 16.30│ 1313.3│
│ 16.34│ 1332.5│
│ 16.38│ 1351.7│
│ 16.42│ 1370.8│
│ 16.46│ 1389.9│
│ 16.50│ 1408.9│
│ 16.54│ 1427.9│
│ 16.57│ 1446.8│
│ 16.61│ 1465.7│
│ 16.65│ 1484.7│
│ 16.69│ 1503.4│
│ 16.73│ 1522.2│
│ 16.77│ 1540.8│
│ 16.81│ 1559.5│
│ 16.85│ 1578.2│
│ 16.89│ 1596.8│
│ 16.93│ 1615.3│
│ 16.97│ 1633.8│
│ 17.01│ 1652.2│
│ 17.05│ 1670.6│
│ 17.09│ 1689.0│
│ 17.13│ 1707.3│
│ 17.17│ 1725.6│
│ 17.20│ 1743.8│
│ 17.24│ 1762.1│
│ 17.28│ 1780.3│
│ 17.32│ 1798.4│
│ 17.36│ 1816.5│
│ 17.40│ 1834.5│
│ 17.44│ 1852.5│
│ 17.48│ 1870.4│
│ 17.52│ 1888.3│
│ 17.56│ 1906.2│
│ 17.60│ 1924.0│
│ 17.64│ 1941.8│
│ 17.68│ 1959.6│
│ 17.72│ 1977.3│
│ 17.76│ 1994.9│
│ 17.79│ 2012.6│
│ 17.83│ 2030.2│
│ 17.87│ 2047.8│
│ 17.91│ 2065.3│
│ 17.95│ 2082.8│
│ 17.99│ 2100.2│
│ 18.03│ 2117.6│
│ 18.07│ 2135.0│
│ 18.11│ 2152.3│
│ 18.15│ 2169.6│
│ 18.19│ 2186.9│
│ 18.23│ 2204.1│
│ 18.27│ 2221.3│
│ 18.31│ 2238.4│
│ 18.35│ 2255.5│
│ 18.39│ 2272.6│
│ 18.42│ 2289.6│
│ 18.46│ 2306.6│
│ 18.50│ 2323.6│
│ 18.54│ 2340.5│
│ 18.58│ 2357.4│
│ 18.62│ 2374.2│
│ 18.66│ 2391.1│
│ 18.70│ 2407.9│
│ 18.74│ 2424.6│
│ 18.78│ 2441.3│
│ 18.82│ 2458.0│
│ 18.86│ 2474.6│
│ 18.90│ 2491.3│
│ 18.94│ 2507.9│
│ 18.98│ 2524.3│
│ 19.02│ 2540.8│
│ 19.05│ 2557.3│
│ 19.09│ 2573.7│
│ 19.13│ 2590.2│
│ 19.17│ 2506.6│
│ 19.21│ 2622.9│
│ 19.25│ 2639.2│
│ 19.29│ 2655.4│
│ 19.33│ 2671.6│
│ 19.37│ 2687.9│
│ 19.41│ 2704.1│
│ 19.45│ 2720.2│
│ 19.49│ 2736.3│
│ 19.53│ 2752.3│
│ 19.57│ 2768.3│
│ 19.61│ 2784.4│
│ 19.65│ 2800.4│
│ 19.68│ 2816.3│
│ 19.72│ 2832.2│
│ 19.76│ 2848.1│
│ 19.80│ 2864.0│
│ 19.84│ 2879.8│
│ 19.88│ 2895.6│
│ 19.92│ 2911.3│
│ 19.96│ 2927.0│
│ 20.00│ 2942.7│
│ 20.04│ 2958.4│
│ 20.08│ 2974.0│
│ 20.12│ 2989.6│
│ 20.16│ 3005.2│
│ 20.20│ 3020.7│
│ 20.24│ 3036.2│
│ 20.28│ 3051.7│
│ 20.31│ 3067.2│
│ 20.35│ 3082.6│
│ 20.39│ 3097.9│
│ 20.43│ 3113.3│
│ 20.47│ 3128.6│
│ 20.51│ 3143.9│
│ 20.55│ 3159.2│
│ 20.59│ 3174.4│
│ 20.63│ 3189.7│
│ 20.67│ 3204.9│
│ 20.71│ 3220.0│
│ 20.75│ 3235.1│
│ 20.79│ 3250.2│
│ 20.83│ 3265.3│
│ 20.87│ 3280.3│
│ 20.90│ 3295.3│
│ 20.94│ 3310.3│
│ 20.98│ 3325.3│
│ 21.02│ 3340.2│
│ 21.06│ 3355.1│
│ 21.10│ 3370.0│
│ 21.14│ 3384.8│
│ 21.18│ 3399.6│
│ 21.22│ 3414.4│
│ 21.26│ 3429.2│
│ 21.30│ 3443.9│
│ 21.34│ 3458.6│
│ 21.38│ 3473.3│
│ 21.42│ 3487.9│
│ 21.46│ 3502.5│
│ 21.50│ 3517.2│
│ 21.54│ 3531.8│
│ 21.57│ 3546.3│
│ 21.61│ 3560.8│
│ 21.65│ 3575.3│
│ 21.69│ 3589.8│
│ 21.73│ 3604.2│
│ 21.77│ 3618.6│
│ 21.81│ 3633.0│
│ 21.85│ 3647.4│
│ 21.89│ 3661.7│
│ 21.93│ 3676.0│
│ 21.97│ 3690.3│
│ 22.01│ 3704.6│
│ 22.05│ 3718.8│
│ 22.09│ 3733.0│
│ 22.13│ 3747.2│
│ 22.17│ 3761.3│
│ 22.20│ 3775.4│
│ 22.24│ 3789.5│
│ 22.28│ 3803.6│
│ 22.32│ 3817.7│
│ 22.36│ 3831.7│
│ 22.40│ 3845.7│
│ 22.44│ 3859.7│
│ 22.48│ 3873.7│
│ 22.52│ 3887.6│
│ 22.56│ 3901.5│
│ 22.60│ 3915.4│
│ 22.64│ 3929.3│
│ 22.68│ 3943.1│
│ 22.72│ 3956.9│
│ 22.76│ 3970.7│
│ 22.80│ 3984.5│
│ 22.83│ 3998.2│
│ 22.87│ 4011.9│
│ 22.91│ 4025.6│
│ 22.95│ 4039.3│
│ 22.99│ 4052.9│
│ 23.03│ 4066.6│
│ 23.07│ 4080.2│
│ 23.11│ 4093.8│
│ 23.15│ 4107.3│
│ 23.19│ 4120.8│
│ 23.23│ 4134.3│
│ 23.27│ 4147.8│
│ 23.31│ 4161.3│
│ 23.35│ 4174.7│
│ 23.39│ 4188.1│
│ 23.43│ 4201.5│
│ 23.46│ 4214.9│
│ 23.50│ 4228.2│
│ 23.54│ 4241.6│
│ 23.58│ 4254.9│
│ 23.62│ 4268.2│
│ 23.66│ 4281.4│
│ 23.70│ 4294.7│
│ 23.74│ 4307.9│
│ 23.78│ 4321.1│
│ 23.82│ 4334.3│
│ 23.86│ 4347.4│
│ 23.90│ 4360.5│
│ 23.94│ 4373.7│
│ 23.98│ 4386.7│
│ 24.02│ 4399.8│
│ 24.06│ 4412.8│
│ 24.09│ 4425.9│
│ 24.13│ 4438.9│
│ 24.17│ 4451.9│
│ 24.21│ 4464.8│
│ 24.25│ 4477.7│
│ 24.29│ 4490.7│
│ 24.33│ 4503.6│
│ 24.37│ 4516.4│
│ 24.41│ 4529.3│
│ 24.45│ 4542.1│
│ 24.49│ 4554.9│
│ 24.53│ 4567.7│
│ 24.57│ 4580.5│
│ 24.61│ 4593.2│
│ 24.65│ 4606.0│
│ 24.68│ 4618.7│
│ 24.72│ 4631.4│
│ 24.76│ 4644.0│
│ 24.80│ 4656.7│
│ 24.84│ 4669.3│
│ 24.88│ 4682.0│
│ 24.92│ 4694.5│
│ 24.96│ 4707.1│
│ 25.00│ 4719.7│
│ 25.04│ 4732.2│
│ 25.08│ 4744.7│
│ 25.12│ 4757.2│
│ 25.16│ 4769.7│
│ 25.20│ 4782.1│
│ 25.24│ 4794.6│
│ 25.28│ 4807.0│
│ 25.31│ 4819.4│
│ 25.35│ 4831.7│
│ 25.39│ 4844.1│
│ 25.43│ 4856.4│
│ 25.47│ 4868.7│
│ 25.51│ 4881.0│
│ 25.55│ 4893.3│
│ 25.59│ 4905.6│
│ 25.63│ 4917.8│
│ 25.67│ 4930.0│
│ 25.71│ 4942.2│
│ 25.75│ 4954.4│
│ 25.79│ 4966.6│
│ 25.83│ 4978.7│
│ 25.87│ 4990.9│
│ 25.91│ 5003.0│
│ 25.94│ 5015.1│
│ 25.98│ 5027.2│
│ 26.02│ 5039.3│
│ 26.06│ 5051.2│
│ 26.10│ 5063.2│
│ 26.14│ 5075.3│
│ 26.18│ 5087.2│
│ 26.22│ 5099.2│
│ 26.26│ 5111.2│
│ 26.30│ 5123.1│
│ 26.34│ 5135.0│
│ 26.38│ 5146.9│
│ 26.42│ 5158.8│
│ 26.46│ 5170.6│
│ 26.50│ 5182.5│
│ 26.54│ 5194.3│
│ 26.57│ 5206.1│
│ 26.61│ 5217.9│
│ 26.65│ 5229.7│
│ 26.69│ 5241.4│
│ 26.73│ 5253.2│
│ 26.77│ 5264.9│
│ 26.81│ 5276.6│
│ 26.85│ 5288.3│
│ 26.89│ 5300.0│
│ 26.93│ 5311.6│
│ 26.97│ 5323.2│
│ 27.01│ 5334.8│
│ 27.05│ 5346.4│
│ 27.09│ 5358.0│
│ 27.13│ 5369.6│
│ 27.17│ 5381.1│
│ 27.21│ 5392.7│
│ 27.25│ 5404.2│
│ 27.28│ 5415.6│
│ 27.32│ 5427.2│
│ 27.36│ 5438.7│
│ 27.40│ 5450.1│
│ 27.44│ 5461.5│
│ 27.48│ 5472.9│
│ 27.52│ 5484.3│
│ 27.56│ 5495.7│
│ 27.60│ 5507.1│
│ 27.64│ 5518.4│
│ 27.68│ 5529.8│
│ 27.72│ 5541.1│
│ 27.76│ 5552.4│
│ 27.80│ 5563.7│
│ 27.84│ 5575.0│
│ 27.87│ 5586.2│
│ 27.91│ 5597.5│
│ 27.95│ 5608.7│
│ 27.99│ 5619.6│
│ 28.03│ 5631.1│
│ 28.07│ 5642.2│
│ 28.11│ 5653.4│
│ 28.15│ 5664.6│
│ 28.19│ 5675.7│
│ 28.23│ 5686.8│
│ 28.27│ 5697.9│
│ 28.31│ 5709.0│
│ 28.35│ 5720.1│
│ 28.39│ 5731.1│
│ 28.43│ 5742.1│
│ 28.46│ 5753.1│
│ 28.50│ 5764.2│
│ 28.54│ 5775.1│
│ 28.58│ 5786.1│
│ 28.62│ 5797.1│
│ 28.66│ 5808.0│
│ 28.70│ 5819.0│
│ 28.74│ 5829.9│
│ 28.78│ 5840.8│
│ 28.82│ 5851.7│
│ 28.86│ 5862.5│
│ 28.90│ 5873.4│
│ 28.94│ 5884.2│
│ 28.98│ 5894.9│
│ 29.02│ 5905.8│
│ 29.06│ 5916.7│
│ 29.09│ 5927.5│
│ 29.13│ 5938.2│
│ 29.17│ 5949.0│
│ 29.21│ 5959.7│
│ 29.25│ 5970.4│
│ 29.29│ 5981.2│
│ 29.33│ 5991.9│
│ 29.37│ 6002.5│
│ 29.41│ 6013.2│
│ 29.45│ 6023.8│
│ 29.49│ 6034.4│
│ 29.53│ 6045.1│
│ 29.57│ 6055.7│
│ 29.61│ 6066.3│
│ 29.65│ 6076.9│
│ 29.69│ 6087.5│
│ 29.72│ 6098.0│
│ 29.76│ 6108.6│
│ 29.80│ 6119.1│
│ 29.84│ 6129.6│
│ 29.88│ 6140.1│
│ 29.92│ 6150.6│
│ 29.96│ 6161.1│
│ 30.00│ 6171.5│
│ 30.04│ 6182.0│
│ 30.08│ 6192.4│
│ 30.12│ 6202.8│
│ 30.16│ 6213.2│
│ 30.20│ 6223.6│
│ 30.24│ 6234.0│
│ 30.28│ 6244.4│
│ 30.32│ 6254.7│
│ 30.35│ 6265.0│
│ 30.39│ 6275.4│
│ 30.43│ 6285.7│
│ 30.47│ 6296.0│
│ 30.51│ 6306.2│
│ 30.55│ 6316.5│
│ 30.59│ 6326.7│
│ 30.63│ 6337.0│
│ 30.67│ 6347.2│
│ 30.71│ 6357.4│
│ 30.75│ 6367.6│
│ 30.79│ 6377.8│
│ 30.83│ 6388.0│
│ 30.87│ 6398.2│
│ 30.91│ 6408.3│
│ 30.94│ 6418.5│
│ 30.98│ 6428.6│
│ 31.02│ 6438.7│
│ 31.06│ 6448.8│
└───────────────┴───────────────┘

TABLE B.

┌───────────────┬───────────────┐
│ Deg.[4] │ Met. │
├───────────────┼───────────────┤
│ 0.2│ 0.3│
│ 0.4│ 0.6│
│ 0.6│ 0.9│
│ 0.8│ 1.2│
│ 1.0│ 1.5│
│ 1.2│ 1.8│
│ 1.4│ 2.1│
│ 1.6│ 2.3│
│ 1.8│ 2.6│
│ 2.0│ 2.9│
│ 2.2│ 3.2│
│ 2.4│ 3.5│
│ 2.6│ 3.8│
│ 2.8│ 4.1│
│ 3.0│ 4.4│
│ 3.2│ 4.7│
│ 3.4│ 5.0│
│ 3.6│ 5.3│
│ 3.8│ 5.6│
│ 4.0│ 5.9│
│ 4.2│ 6.2│
│ 4.4│ 6.5│
│ 4.6│ 6.8│
│ 4.8│ 7.1│
│ 5.0│ 7.4│
│ 5.2│ 7.6│
│ 5.4│ 7.9│
│ 5.6│ 8.2│
│ 5.8│ 8.5│
│ 6.0│ 8.8│
│ 6.2│ 9.1│
│ 6.4│ 9.4│
│ 6.6│ 9.7│
│ 6.8│ 10.0│
│ 7.0│ 10.3│
│ 7.2│ 10.6│
│ 7.4│ 10.9│
│ 7.6│ 11.2│
│ 7.8│ 11.5│
│ 8.0│ 11.8│
│ 8.2│ 12.1│
│ 8.4│ 12.4│
│ 8.6│ 12.6│
│ 8.8│ 12.9│
│ 9.0│ 13.2│
│ 9.2│ 13.5│
│ 9.4│ 13.8│
│ 9.6│ 14.1│
│ 9.8│ 14.4│
│ 10.0│ 14.7│
│ 10.2│ 15.0│
│ 10.4│ 15.3│
│ 10.6│ 15.6│
│ 10.8│ 15.9│
│ 11.0│ 16.2│
│ 11.2│ 16.5│
│ 11.4│ 16.8│
│ 11.6│ 17.1│
│ 11.8│ 17.4│
│ 12.0│ 17.6│
│ 12.2│ 17.9│
│ 12.4│ 18.2│
│ 12.6│ 18.5│
│ 12.8│ 18.8│
│ 13.0│ 19.1│
│ 13.2│ 19.4│
│ 13.4│ 19.7│
│ 13.6│ 20.0│
│ 13.8│ 20.3│
│ 14.0│ 20.6│
│ 14.2│ 20.9│
│ 14.4│ 21.2│
│ 14.6│ 21.5│
│ 14.8│ 21.8│
│ 15.0│ 22.1│
│ 15.2│ 22.4│
│ 15.4│ 22.7│
│ 15.6│ 22.9│
│ 15.8│ 23.2│
│ 16.0│ 23.5│
│ 16.2│ 23.8│
│ 16.4│ 24.1│
│ 16.6│ 24.4│
│ 16.8│ 24.7│
│ 17.0│ 25.0│
│ 17.2│ 25.3│
│ 17.4│ 25.6│
│ 17.6│ 25.9│
│ 17.8│ 26.2│
│ 18.0│ 26.5│
│ 18.2│ 26.8│
│ 18.4│ 27.1│
│ 18.6│ 27.4│
│ 18.8│ 27.7│
│ 19.0│ 28.0│
│ 19.2│ 28.2│
│ 19.4│ 28.5│
│ 19.6│ 28.8│
│ 19.8│ 29.1│
│ 20.0│ 29.4│
└───────────────┴───────────────┘

Footnote 4:

The degrees refer to the centigrade thermometer.

TABLE C.

┌─────────────┬─────────────┬─────────────┬─────────────┬─────────────┐ │ Approximate │ 0° │ 15° │ 40° │ 55° │ │ Height. │ │ │ │ │ ├─────────────┼─────────────┼─────────────┼─────────────┼─────────────┤ │ 200│ 1.2│ 1.0│ 0.6│ 0.4│ │ 400│ 2.4│ 2.2│ 1.4│ 0.8│ │ 600│ 3.4│ 3.2│ 2.0│ 1.2│ │ 800│ 4.5│ 4.3│ 2.8│ 1.7│ │ 1000│ 5.7│ 5.3│ 3.4│ 2.2│ │ 1200│ 7.0│ 6.4│ 4.2│ 2.6│ │ 1400│ 8.2│ 7.6│ 4.8│ 3.0│ │ 1600│ 9.2│ 8.8│ 5.6│ 3.4│ │ 1800│ 10.4│ 9.8│ 6.3│ 3.8│ │ 2000│ 11.6│ 11.0│ 7.0│ 4.2│ │ 2200│ 12.8│ 12.1│ 7.6│ 4.6│ │ 2400│ 14.0│ 13.3│ 8.4│ 5.1│ │ 2600│ 15.2│ 14.4│ 9.2│ 5.6│ │ 2800│ 16.5│ 15.6│ 10.0│ 6.2│ │ 3000│ 17.7│ 16.8│ 10.8│ 6.6│ │ 3200│ 10.1│ 18.0│ 11.5│ 7.0│ │ 3400│ 20.5│ 19.3│ 12.4│ 7.7│ │ 3600│ 21.8│ 20.4│ 13.4│ 8.2│ │ 3800│ 23.1│ 21.6│ 14.3│ 8.7│ │ 4000│ 24.6│ 22.9│ 15.1│ 9.4│ │ 4200│ 25.9│ 24.3│ 15.9│ 10.1│ │ 4400│ 27.5│ 25.8│ 16.9│ 10.8│ │ 4600│ 28.9│ 27.1│ 18.0│ 11.5│ │ 4800│ 30.4│ 28.4│ 19.0│ 12.1│ │ 5000│ 31.8│ 29.8│ 19.9│ 12.7│ │ 5200│ 33.0│ 31.0│ 20.8│ 13.3│ │ 5400│ 34.3│ 32.4│ 21.7│ 13.9│ │ 5600│ 35.7│ 33.7│ 22.6│ 14.5│ │ 5800│ 37.1│ 35.0│ 23.6│ 15.1│ │ 6000│ 38.5│ 36.3│ 24.6│ 15.7│ └─────────────┴─────────────┴─────────────┴─────────────┴─────────────┘

TABLE D.

┌───────────────┬───────────────┐
│ Barometrical │ Metres │
│ Height │ │
├───────────────┼───────────────┤
│ 15.75 │ 1.71 │
│ 17.72 │ 1.39 │
│ 19.68 │ 1.11 │
│ 21.65 │ 0.86 │
│ 23.62 │ 0.63 │
│ 25.59 │ 0.42 │
│ 27.56 │ 0.22 │
│ 29.53 │ 0.03 │
└───────────────┴───────────────┘