THE LEVEL BOOK FOR PLOTTING THE SECTION.

(Datum 100 feet below the station A.)

The several differences of the sums of the back and fore sights, of the sums of the rises and falls, and of the last reduced level and the datum, exactly agreeing, proves the accu

racy of the arithmetical operation in the preceding level book, all these differences being 49.33 feet, which is the height of B above A, in the section Plate II.

It is advisable for the surveyor to reduce the levels in the field as he proceeds, as it will occupy very little time and can be easily done while the staffman is taking a new position. The surveyor will thus be enabled to detect with the eye if he is committing any glaring error; for instance, inserting a number in the column of rises, when it ought to be in that of falls, the surface of the ground at once reminding him that he is going downward instead of ascending.

It is seldom the case in practice that the instrument can be placed precisely equi-distant from the back and fore staves, on account of the inequalities of the ground, ponds, &c.; it would appear, therefore, to be necessary, to make our results perfectly correct, to apply to each observation the correction for curvature and refraction as explained in Art. (7): this, we believe, is seldom done, unless in particular cases where the utmost possible accuracy is required, on account of the smallness of such correction, as may be seen by referring to the table at the end of the book, where this correction for 11 chains is shewn to be no more than 10 part of a foot; and as the difference in the distances between the instrument and the fore and back staves can in no case equal that sum, it is evident that such correction may be safely disregarded in practice. Besides it is not necessary to have the level placed directly between the staves while making observations, as it is frequently inconvenient to do so, for reasons just given, nor does a deviation from a line of the staves, in this respect, in the least affect the accuracy of the result.

The distances in the sixth column of the level book are assumed to be horizontal distances, and in measuring them, care should be taken that they are as nearly such as possible, or they must be afterwards reduced thereto, otherwise the section will be longer than it ought to be. For the purpose of assisting the surveyor in making the necessary reduction from the hypothenusal to the horizontal measure, when laying down the section, we annex the following table, shewing the reduction to be made on each chain's length for the following quantities of rise, as shewn by the reading of the staves.

TO DRAW THE SECTION.

The levels being reduced, as well as the distances corrected, where required, the surface line may be represented in the form of a section, as shewn in the upper part of Plate II. The vertical and horizontal scales of a section are seldom the same, for the reason assigned in Art. (14.), which produces a caricatured representation of the surface of the section; the vertical scale being so much greater than the horizontal, shews the depths of the cuttings and embankments, required in the execution of railways, canals, &c., with greater clearness than if both scales were alike. The plans and sections of projected work, deposited in the Private Bill Office, to obtain the sanction of the legislature, are mostly drawn to a scale of 13 inches to one mile horizontal and one hundred feet to one inch vertical.

To make the section expeditiously, first draw the horizontal line EF, as the datum to which the levels were reduced, take any point E as the starting point, place the feathered edge of the horizontal scale against E F and prick off the several distances in the column headed "Distances," that is, 5·19, 13.15, 15.42 chains, &c., then draw all the perpendiculars by means of a parallel ruler, or by a T square if the paper is properly fixed on a drawing table; and lastly from the vertical scale prick off all the reduced levels or vertical heights corresponding to the several distances, connect the points and the section will be made, after which, write, in vertical lines, the several remarks, as crossing of roads, rivers, &c., that appear in the last column of the level book. The horizontal line AD, called the "datum from bench mark at A," shews the variations of the surface of

the section above and below the point A, from which it was originally plotted by Mr. Simms, his level book being adapted thereto.

CROSS LEVELS.

When a road or river crosses the line of a railway, or canal, cross levels are principally taken to shew the nature of the surface of the ground, both with respect to improving the main line, if possible, and to shew the approaches of cross roads to the viaducts, when required, and the depth and length of cutting, or the height and length of embankment that will be required, where the main line is either crossed on the level, or over or under by viaducts. The heights of the cross section is usually taken at every 1 or 2 chains length to the distance of 8 or 10 chains on each side of the main line; but if the cross road have a regular slope, it will be sufficient to shew the inclination of the slope, which may be done by a single setting up of the level and staves. The following notes will shew the method of taking a cross section, recollecting always to take the levels from the right to the left of the forward direction of the main line, lest the section, through mistake, should be plotted in the wrong direction.

CROSS LEVELS ON ROAD AT 92.15 CHAINS.

The cross section may be plotted on the same scale as the main section; but some engineers adopt a larger one. From what has been already shewn, the student can have no dif ficulty in plotting from the preceding notes.

PARLIAMENTARY PLAN AND SECTION.

These differ little from what is shewn on Plate II, only in the former the fields, &c., are numbered with an accompanying book of reference to proprietors' names, &c.; and the cross sections are added in the latter, and the gradients put on, as described at page 153. See Working Section, Plate III., and the Standing Orders of the two Houses of Parliament, in No. 62 of Weale's Series.

WORKING SECTION.

When the works of a railway, canal, &c., have to be carried into execution, the section must be more minutely taken than would be required in the preceding cases; it is then called a working section. The following are the field notes for such a section. Plate III. shews the section of the ground and railway at the extreme end of the line, where the distances terminate at 1103-77 chains, or 133 miles and 3·77 chains. The student will be able to plot the section from the following level book and the accompanying directions. In taking levels for this section, the back and fore sights are not very far distant from each other, and the surveyor will frequently be able to make a number of observations at each setting up of the instrument both in the back and forward directions, the back staff being repeatedly removed nearer the instrument, where the ground varies, and the fore staff also repeatedly removed farther from it in this manner from seven to ten observations may be obtained at one setting up of the instrument, if required. In the following level book, it will be seen, that it seldom occurred that only one back and one fore sight was obtained at each setting up of the instrument: at the first setting up four forward sights were observed; thus the first back sight was 4.47 and the corresponding fore sights 4.53, 9.22, 5·07, &c. Here the first fall is obtained in the usual manner, that is, by taking the difference between 4.53 and 4.47; the next fall is obtained by taking the difference between the first and second fore sights, that is, between 9.22 and 4.53, this difference is 4.69, which is a fall, because the latter fore sights is the greater; the third difference between the second and third fore sights is a rise, because the latter fore sight in this case, is the less: and so on till we come to the next back sight 6.36, when its corresponding fore sight 1.87 is taken from it, as previously shewn, and the difference placed in the column of rises. The column of reduced levels is obtained as in the preceding examples.

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UNIVERSITY