DRAINAGE 



161 



give an outlet for smaller canals and for needed farm 

 and roadside ditches. 



A general map thus made was published in a pamphlet 

 and has been of great use to the counties, townships and 

 farmers co-operating in the drainage of these lands. 

 A copy of a portion of this map is shown in Figure 70, 

 page 167. In some low areas of much less size, even in 

 single fields, it is necessary to take levels at various 

 points, or, as the engineer says, " cross section the field," 

 so as to map the 

 contour or ele- 

 vation of the en- 

 tire area and thus 

 decide where 

 drains are neces- 

 sary and prac- 

 tical. The level is 

 a necessity also 

 in determining 

 the rates of fall 

 and the grades 

 that should be 

 given to the 

 ditch or tile 

 drain where the 

 grades are so 

 nearly level that 

 they must be 

 very accurately 

 made. It is 

 often needed, while constructing the drain, to see 

 that the bottom of the ditch is at the right depth 

 at the various points. For draining small areas it is 

 often unnecessary to use an expensive instrument, as 

 the home-made instruments shown in Figures 59-61, 

 may serve the purpose. 



Figure 61 represents a simple form of home-made leveling in- 

 strument which is useful where great accuracy is not required. 

 P is a tube of tin or of gas pipe. At either end of the tube a 

 glass tube, G, a few inches long, is inserted. Colored water 

 is poured in so as to rise nearly to the corks in the tubes. A 

 front and a back sight, as SS, or another form of sight, may 

 be adjusted so as to be easily placed level with the top of the 

 colored liquid in either glass tube. At H is shown a form of 

 joint in the stem connecting the level with the tripod. With 

 this the instrument can be placed so nearly level that the 

 water stands at the point desired in each glass tube. This level 

 does not require adjusting, but it is not accurate for long dis- 

 tances on drains which are nearly level, though in the hands 

 of a careful man it may be found useful under circumstances 

 where a better instrument is not available. 



