2.97 3.39 0.42 105600 290.38 3.39 5.51 2.12 105700 288.26 5.51 7.67 2.16 105800 286.10 5.41 6.68 1.27 105827 284.83 Edge of ditch. 6.68 8.56 1.88 105832 282.95 Bottom of ditch. 2.48 8.56 6.08 6.30 105837 285 43 | Top of bank. 6.08 12.38 4.34 | 105854 | 279.13 | Foot of bank. 12.38 16.72 0.62 274.79 2.86 2.04 2.66 3.82 105900 274-17 2.66 6.48 2:38 105940 270.35 | Edge of ditch. 6.48 8.86 105944 267.97 Bottom of ditch. 8.86 6.00 1.58 Top of bank. 1.69 13.3411.65 1.15 11.65 12.80 0.51 106110 257.77 Edge of ditch. 106200| 256.62 0.27 4.35 0.50 3.85 106359 259.74 Top of bank. 3.74 4.31 0.57 2.45 0.57 3.02 0.41 Bottom of side drain. Centre of parish road. 106415 259.67 Top of bank. 0.99 1.40 1.43 106430 256.81 | Foot of bank. 1.40 2.83 1.58 106500 255.38 2.83 4.41 0.07 106600 253.80 5.14 8.43 3.29 108400 341.49 Edge of bank. Edge of ditch. 109500 264.46 In taking levels for a minute section where the observations must be very numerous, and consequently the back and fore sights not very far from each other, the observer will frequently be able to make a number of observations at each setting up of the level at one side of his line, so that his instrument may be about equally distant from his back and fore observations. Due attention to this will save much time and labour, and experience will enable the surveyor at a glance to see where he can set up his level at every remove forward with the greatest advantage. Upon looking down our field notes above, it will be seen it seldom occurred that only one back and one fore sight was obtained at a setting up of the level, and this only took place where the ground was very steep: by the first setting up of the instrument four forward sights were observed, and of course as many back ones; thus the first back sight was 4.47, the corresponding fore sight 4.53; this latter number was also placed as the back sight for the next observation, which was 9.22; this number was in like manner placed as the back sight for the next forward observation, 5.07, which also became the back sight for the last forward observation we could obtain at that setting up of the instrument, namely, 0.24: it should here be remarked that there was a necessity to place each forward reading as a back observation to the next forward reading, otherwise the difference of level between each point of observation would not have been obtained without more arithmetical work; the numbers otherwise only show the difference of level between each and the first point of observation; besides, by this arrangement, the whole section is continuous, however numerous the intermediate observations may be, and having the distances opposite, the whole can be plotted off with facility. The columns of Rise and Fall need no observation after what has already been said upon this subject. The column of distances denotes the continuous measurements from the commencement, Gunter's chain being the unit employed. Our notes commence at the 1033rd chain, and terminate with the end of the work, 1103 chains and 77 links, which we consider an ample extract for the purposes of the student. The column headed "Reduced Levels" contains the height of each point of observation above the datum line, which in this case was Trinity high-water mark, London Bridge: these numbers are obtained by adding the "rises" and subtracting the "falls" from the preceding reduced level, which in our notes commence with 270.72 feet. THE SECTION. SEE PLATE III. The datum line must be drawn, every chain should then be pricked off and the perpendiculars erected; the chains or stakes should then be numbered beneath the datum line, to prevent mistakes, and just above the datum line the height of the surface at each stake should also be inserted; then the said heights can be pricked off upon the perpendiculars respectively, and the intermediate heights plotted from the field notes without fear of error, which otherwise, without great care, would be likely to occur in consequence of so many points falling near to each other, unless the scale be very large: the horizontal scale of the example is 1 inch to 5 chains, and the vertical scale 1 inch to 25 feet. Having drawn the undulating line of the surface through these points upon the perpendiculars, the gradients or intended line of railway may next be laid down; the extreme left-hand point was given, being the level of the rails at the point of junction with another line. The railway is represented by two parallel lines, the upper one being the upper surface of the rails, and the lower one the bottom of the ballasting or formation level, being 2.25 lower than the surface of the rails: for a short distance the line is level, then it rises at the rate of 20 feet per mile, for the two-fold object of diminishing the great cutting and of getting sufficiently high over the road at stake 1064, to allow (with the lowering the surface of the said road a small quantity) of sufficient headway for the public carriages to pass under the railway from this point the line falls at the rate of 20 feet per mile for a considerable distance, the object being to get as low down as possible further to the eastward, where there was to be a considerable embankment, and by these means such embankment was reduced in dimensions; and furthermore, the earth from the cutting to the right of the road was to be taken eastward to form the said embankment, and therefore the down-hill gradient was favourable for carrying on the work as well as for the drainage of the cutting. Part of the earth from the large cutting was also to be taken to the eastward; the ascending gradient, as far as the bridge, was unfavourable for this purpose, so far, however, as the bringing out the bottom of the cutting, the upper part being brought down by means of inclined planes the ascending gradient was unavoidable in this case, but by judiciously working the excavation, little : |