LEVEL-BOOK.

The projected Railway from the Grand Junction Railway to Abbergelly. (See Section, Plate XIII.)

From the preceding Level-Book the method of proceeding, to any extent, may be sufficiently seen, with the help of the following remarks on the method of obtaining the reduced levels, at the same

NOTE. - That wheresoever the back or fore-sight exceeds the usual height of the levelling staff, that staff must be understood to have been placed on the top of a gate or rail-post, and its height afterwards measured, and added to the reading of the levelling-staff.

(16.) Remarks on obtaining the reduced levels, &c. in the preceding Level-Book, &c.

The method of casting the Level-Book referred to, is the same, with respect to the back and fore-sights, as that given in Art. 10, page 353. These castings must be first done, and the intermediate ones, in the column marked Inter., afterwards.

The datum line is assumed 100 feet below the first station or commencement of the levels: this number is placed first in the column of reduced levels. The back-sight is 2.20 feet, and its corresponding fore-sight 13.40, which shows that the ground falls; the difference of these, 11.20, is put in the column marked Fall, which, being subtracted from the datum 100 feet, leaves *88.80, which is put under the datum, in the column marked Reduced Levels, being marked with an asterisk or cross, and shows the height of the fore-staff above the datum line. The same remarks will apply to the back and foresights 1.80 and 10.10, and to 7.30 and 1.80, observing that in the latter case the ground rises, and the difference must be put in the column marked Rise, and added to the next preceding reduced level that is marked thus *.

The difference between the first back-sight 1.80, and the first intermediate one 10.16, is 8.36, which shows that the ground falls; it is therefore put in the column of falls, under 11.20, and subtracted from the next preceding number in the column of reduced levels, marked thus*, and the remainder 80.44 is then put in that column. In like manner all intermediate differences are added to, or subtracted from, the next preceding reduced level number marked thus*, accordingly as they stand in the column marked Rise or Fall.

The proof of the accuracy of the work is by taking the differences of the sums of the back and fore-sights, and of the first and last reduced levels, which differences must be equal: thus, in the latter portion of the Level-Book, the sums of the back and fore-sights are 135.18 and 80.24, the difference of which is 54.94; and the first and last reduced levels are 123.06 and 178.00, the difference of which is also 54.94, which shows the work to be right.

Mr. Terry's Method of checking the Accuracy of Levels.

(17.) The method of keeping the Level-Book, adopted by Mr. J. E. Terry, many years superintendent of surveys for an eminent railway engineer, is the same as the preceding, with the exception of the second column for the intermediate sights, which is introduced to make it clearer to the student.

Mr. Terry recommends two levellers, with separate instruments placed at a short distance from one another, to read from the same staves at the same time, and frequently to compare their reduced levels, correcting the errors immediately, if any occur; thus completely obviating the numerous mistakes that arise, in extensive operations, through wrong readings, wrong entries in the Level-Book, &c.; since the method of proving the Level-Book by differences, only proves that the reduced levels are correct with respect to the back and fore-sights entered in the book, and not the accuracy of the actual levels.

This system has been frequently put in practice, under Mr. T.'s direction, with the most favourable results, and the author would therefore recommend it.

PLOTTING THE MAIN SECTION.

(18.) First draw the datum line, as in plate XIII, from a scale of 5 chains to an inch; lay off the 4, 1, 4, and 1 mile distances, repeating them after every successive mile, and marking the miles 1, 2, 3, &c. Now lay off on the datum line, from the column of distances in the Level-Book, the several distances up to the first mile; do the same between the second and third mile, between the third and fourth, &c., the distances always beginning afresh from the end of every mile in the Level-Book. Draw lines perpendicular to the datum line at the end of every mile, at least twice the height of the datum line, and parallel to these perpendiculars draw lines in pencil, of the height given in the column of reduced levels, and at their corresponding distances, from a scale of 50 feet to an inch; and through the upper extremities of these lines draw the section, carefully noting the roads, paths, rivers, brooks, woods, &c., as given in the column of remarks in the Level-Book, which, it will be seen, are copied in the plate.

CROSS LEVELS.

(19.) Cross levels are taken principally on roads crossed by the line of railway, to show the position of the surface of the ground, partly with a view, if possible, of improving the main line, and partly to show the nature of the approaches of the cross-roads to the viaducts, where required, and the quantity in respect to depth and length of cutting, or of height and length of embankment required, where the main line is crossed either over or under by viaducts, or on the line of the rails.

The heights in the cross-section are usually taken at every chain's length to the distance of 10 chains on each side of the line, as will be seen in the following notes; remembering always to take the levels

from the left to the right of the onward direction of the main line, otherwise serious errors might arise by plotting the section in the wrong direction.

Cross Levels No. 3., on Road from Winston to Mold.

The cross-section is usually plotted on the same scale as the mainsection; some engineers adopt a larger one. The student can have no difficulty in plotting from the preceding notes. (See plate XIII, Cross-section, No. 3.)

(20.) THE APPLICATION OF THE CORRECTION FOR CURVATURE AND REFRACTION IN LEVELLING.

In the preceding examples on levelling, the application of the correction for curvature and refraction has been avoided, by assuming that the levelling instrument was always placed in observing, at or near the middle point between the staves; thus making the errors arising from these causes to correct each other. (See note to Art. 11.) But in a case where a very long sight is taken in one direction, as, for instance, the fore-sight, the back-sight at the same time being a very short one, these corrections must be strictly attended to, as in the following

Example. In taking levels for a projected railway, a swamp of 148 chains in width occurred in the line; the level was placed at the edge of the swamp, the back-staff was placed one chain behind the instrument, on the level of the swamp, the reading thereon being