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 difficulty 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.

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.

This method of keeping the level book is adopted by many surveyors; the accuracy of the castings is proved by taking the difference of the sum of the back sights and the sum of the last fore sights, leaving out all the intermediate sights, and the difference of the last reduced level, and the same brought forward, and when these differences agree, the castings are correct. Some put the intermediate sights in a separate column, in this case a somewhat different method of casting is adopted :-I have given this method at page 357 of my system of levelling in the ninth edition of Nesbitt's Surveying.