such banks must be lined with clay and a core of clay and boards 
laid down from the proposed wator level to the impervi ous sub- 
stratum. 
 
                               Material 
 
        The most suitable material for construction of dams is one 
 part of clay to two or three parts of sand and gravel. Pure clay 
 and gumboes have a tendency to become distorted. Got the material 
 for the dam as nearby as possible but do not use materials such as 
 gravel and sand wuithout the proper mixture of clay. Mix the clay 
 and gravel and place a foot of the riaterial da.n at a timc. This 
 should then be packed down. The other layers can be added until 
 the dam is of the desired height. 
 
                                Height 
 
        Provide for plenty of clearance from the desired l1ovel of 
 water to the top of the dam. Four feet is the minimum. The de- 
 sired depth of v'ater can be regulated by a sluiceway. 
 
                                 Width 
 
        Dams must be massive cnough to withstand the water pressure 
 and not be susceptible to leakage or overturning. Widths on top 
 should not be less than six feet for dams of a huight of six feet 
 or less, and should not be less tlnn ten feet for dams of a height 
 of ton feet or mpre. Bottom width should be at least five times 
 the height of the dam. Aillowancos for settling, etc., should, be 
 made in addition. Clay ,ill settle somewhat. 
 
                                Slopes 
 
       The upstream side of the dam should be slopoed Ai th a 3 or 
a 4 to 1 angle. That inoans for every vertical foot there should 
be a horizontal distance of three or four feet. The do;.'nstreamn 
side may be somewhat steeper with a slope of 2 to 1. 
 
                                 Core 
 
       It is wvell to dig a ditch from six to eight feet w-ide down 
to the impervious substratum. This core should fall below the 
highest point in the dam. The dovwnstroam side of this ditch should 
be lined with a double layer of 2" plank, w7hich should extend to the

top of the dam. The remainder of the trench must be filled with clay 
which should be -ell puddled and packed. The core prevents excess 
seepage beneath and through the dam and is always desirable regardless 
of soil conditions. In very sandy coils the cores should extend 
into the ground twice the height of the dam. In clay soils they 
can extend only to one half the height of the dam. W'hero w ator is 
encountered, tw o layers of 3" planking driven into the ground is 
sufficient to step seepage. A ditch is then unnecessary. Plow 
furrows every five foot help to join the damn and the foundation 
and prove a help in stopping seepage.