LEVELLING. 



on a temporary staff will fall in with most requirements in 

 connection 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 the staff or support e. The 

 brass screw d serves to adjust the level as required. 



There are two eyesights, a and 6, the latter being a square 

 opening, with a fine hair wire crossing it in the middle. 



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 



e 



FIO. 2. 



METHOD OF LEVELLING. 



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 /, has been rendered horizontal by means of 

 the adjusting screw, an assistant at each of the stations a 

 and ft, 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 at 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 ft 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 



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