LEVELLING AND TOPOGRAPHY. 53 



Bench-marks are permanent objects whose eleva- 

 tions are determined and recorded for future reference. 



Levelling Instruments , 



The engineer's spirit-level in some of its more or 

 less expensive forms is the most accurate instrument 

 and the one with which the most rapid work can be 

 done. Telescope-levels are sufficiently low in price, 

 so that it is not wise to use the poor and cheap kind of 

 levels which are recommended by many as sufficiently 

 accurate for drainage purposes, especially where the 

 work to be done is of any magnitude. The fact that 

 most of our drainage work requires the utmost accuracy 

 attainable is not appreciated by many. The care and 

 adjustment of the instrument should be learned and can 

 usually be done with the aid of little manuals which the 

 instrument-makers furnish. Some of the levels have a 

 graduated circle, by means of which horizontal angles 

 can be measured and made use of in platting the lines 

 along which levels are taken. For this work, however, 

 a compass either attached or separate from the level is 

 much to be preferred. 



The rod is a necessary companion to the level. 

 There is a great variety of forms, each one having its 

 advocates. Some form of the speaking-rod or self-read- 

 ing rod is to be preferred to any form of the target rod 

 for the work herein described. It is capable of being 

 used more rapidly, it makes the levelman more inde- 

 pendent of his rodman because he takes his own read- 

 ings, and in making cross-sections it can be used instead 

 of a tape-line. The divisions and colors should be so 

 displayed as to be clearly seen and easily read. For 



