pronounced and indicated at various points occurring in its series of primary and subsidiary ridges and valleys, or watershed and watercourse lines. This double or combined system of lines constitutes a method of naturally subdividing the surface of any portion of the earth, however large or small. Commencing with the largest continents; these may be naturally subdivided into a series of extensive catchment areas, or river basins, within each of which all rainfall follows an independent course to the sea. The limits of these catchments are main watershed lines or ridges, sometimes marked by mountain ranges and sometimes hardly distinguishable to the naked eye on account of the flatness of some plateau, but in all cases definite lines. These extensive catchments again can be subdivided into a series of secondary catchments, whose limits consist of portions of the main watershed lines and secondary ridges branching from the former. This process of successive subdivision of catchment areas may be continued until the resulting portions become as small as English combes, and possibly again until the parts become microscopic, the limiting lines being always watersheds, main or subsidiary, or both in various orders, primary, secondary, tertiary, and so forth. Commencing afresh with the large rivers that discharge into the sea from any continent; the main courses of these are the main watercourse lines of the continent; their tributaries are the secondary watercourse lines branching off from the former, the affluents of the tributaries form the tertiary watercourse lines, and the subdivision may be carried on by these ramifications until the last watercourse lines are merely excessively small rills in which water may only occasionally run in minute quantity. The sub

division of catchment areas is illustrated in Plate VII. The points of divergence of the watershed lines, the points of convergence of the watercourse lines, the points of marked change of inclination occurring both along the watershed and watercourse lines, such as peaks, passes, and hollows, and the levels of table-lands, plains, and lakes, are thus the points that most distinctively indicate the natural features of the country. The principal or most marked among these are the points whose levels are observed in order to carry out the object first mentioned. The position of these points is determined by surveying operations, although some additional measurements are sometimes needed in conjunction with the levelling.

2. Contours, or contour lines, are lines of equal relative height on the earth's surface; the edge of a lake is an instance of a natural contour line, and the edges of imaginary lakes having various successive depths of water of equal decrement represent the artificial contour lines conventionally used in plans to indicate detail in form of surface. The difference of elevation between any two contours on a plan is a constant quantity, generally some round number in feet, fixed in accordance with the declivities of the country and the scale of the plan. Contour lines are almost invariably at right angles both to the principal watershed lines and to the principal watercourse lines wherever they exactly happen to cross them, the exceptions being where they become coincident with them, as at very sharply-defined ridges or hollows of equal height or equal depth. See Plate VIII. 3. Lines of distant bench-marks.-These are generally undertaken by the national survey department of the country, and executed with superior instruments and great

precision for purposes of public utility and reference. In very extended open country the course followed is fixed by convenience and is frequently along main valleys, or in a series of lines connecting the principal towns. The results are a series of correctly marked bench-marks, and a list of them showing their position and height. The experience gained from such operations also forms a guide to ordinary levelling and a means of estimating causes and sources of error; a subject that will be treated in a subsequent section.

4. Flying levels.-These are taken at any detached points chosen in connection with some special object, more generally some proposed work of communication, and serve as a basis for preliminary designs. The binding points of such designs are therefore those whose levels are specially required; they are generally practically useful points at or near towns, at various crossing points of rivers, saddles of ridges, points of abrupt change in the general rise or fall of the country, and proposed points of junction with existing lines of communication. Such levels are generally rapidly and rather roughly determined; they may either be recorded on plans or in the form of rough sections on a very small scale.

5. Complete levels for final designs.-A series of detailed levels sufficiently complete for purposes of final design for works of communication should consist firstly in a fully corrected set of levels for the points previously determined for the preliminary design; secondly, in an amplification of the same by levels of intermediate and lateral points; and thirdly, in sets of contour levels over such parts of the country as are most specially affected by the intended works or are most likely to affect the

proposed construction and design by their form of surface. Canals and works involving the collection, storage, and distribution of water are more specially affected by inaccuracy of level. The greater portion of such levels are most generally recorded in the form of longitudinal sections, with a series of short or cross sections; the remainder being recorded as contours on portions of the plan.

Under this, as well as the foregoing four heads, the purposes of the actual design or intended object can alone serve as a guide to the extent of work and amount of detail required in the series of levels. The scope of such operations may vary from a few hundred feet to a few hundred miles, and the amount of detail required may also vary enormously.

The nature of the country affects levelling operations to a great degree; the use of the Gravatt level being confined principally to level and moderately undulating localities, the Eckhold principle and instruments of the inclined or A-level type being suited to inclined ground, and the portable reflecting-level being necessary in steep and precipitous country. These three last considerations, as well as the five previously given, and the variety of extent and detail, constitute the practical distinctions in levelling operations of various kinds.

Section 3. LEVELLING OPERATIONS.

Levelling in Moderately Even Localities.

The principal part of all works of engineering, manufacture, agriculture, and trade being carried on in the plains or level portions of any country, the greater portion of levelling operations necessarily also happens to be carried out in similar localities in order to meet

the wants of those residing there. Telescopic levelling instruments and graduated level-staves of the Gravatt type are the appliances most commonly used under such circumstances; the 14 or 16-inch level—that is, an instrument with a 16-inch telescope-being the size adopted for ordinary purposes, and the 18-inch level being used wherever difference of level becomes more important and greater accuracy is required, as for canals in very flat country. The former instrument generally should have a telescope with a magnifying power of about 16 to 18 times, the latter about 18 to 20 times. In either case the level should be as long as the telescope admits of, and should be sensitive, that is of rather flat curvature. It is, however, better in all cases to have a telescope of higher magnifying power-for the former 20 times, for the latter 25 times-for this causes only one drawback, that very short sights cannot be taken with it, while the advantages of being able to take long sights and get over ground faster, and that of using the instrument with additional wires as a telemeter, are relatively important.

It is most usual that surveying operations precede those of levelling, so that the points at which levels are required are shown on some plan, and may be also marked on the ground by whites or pegs ready for the leveller; practically however it often happens that this is only done for a certain number of points, and that additional measurements have to be chained for determining the position of many more; under such circumstances observation of distance with the telescope-wires effects a great saving of time, and within certain distances is quite as correct as chaining. (See paragraphs on Telemetry, page 16.)