=Levelling.=—In the carrying out of drainage operations in connexion with the formation of new plantations, but particularly where the land is nearly level, one of the most necessary implements is the spirit-level or theodolite. There are many other operations, as well as in connexion with draining, where the level and the knowledge of its working are all-important for the forester. Thus in the formation of roads and paths, levelling of ground inequalities, lake and pond forming, etc., the level will be found an almost indispensable instrument—in fact, it cannot well be done without. Of levelling instruments there are various descriptions, but the simplest of any is the ordinary spirit-level, it being cheap, easily carried about in the pocket, and when erected on a temporary staff will fall in with most requirements in connexion with general forest work. The illustration (Fig. 1) will give a good idea of the simple instrument. When in use the spirit-level is fixed in a frame of brass, the whole being screwed into a staff or support, _e_. The brass screw, _d_, serves to adjust the level as required. There are two eyesights, _a_ and _b_, the latter being a square opening, with a fine hair wire crossing it in the middle. [Illustration: FIG. 1. SPIRIT-LEVEL.] [Illustration: FIG. 2. METHOD OF LEVELLING.] The relative heights of a series of points are obtained by means of their vertical distances from others which, on the supposition of the earth being a sphere, are equally distant from its centre, and these, which are called level-points, must be found by an instrument constructed for the purpose—spirit-level, theodolite, etc. Generally choice is made of any convenient stations, _a, b, c, d_, on the line of operation (see Fig. 2), and the distances between them are determined by actual ad-measurement. The instrument is then set up and adjusted at, or near, the middle of the interval between every two such points in succession. When the level thus placed, as at _f_, has been rendered horizontal by means of the adjusting screw, an assistant at each of the stations _a_ and _b_, holding what is called a station-staff in a vertical position, moves an index along the staff, up or down, as dictated by the observer of the level, till it coincides with the intersecting wire as seen in the eyesight or telescope. The points thus determined on the stave are represented by _e_ and _g_, and these are termed level-points, or points equally distant from the centre of the earth. Therefore the heights _a, e_, and _b, g_, being read on the graduated staves, the difference between them will give the relative heights of the ground at _a_ and _b_. Similar processes are repeated with respect to the points _b_ and _c_, and _c_ and _d_, the instrument being placed at _i_ and _m_, midway between them. Usually the heights _b g, c l_ and _d n_ are inserted in a column headed “Foresights,” and the heights _a e, b h_, _c k_ and _d n_ in a collateral column headed