216 IRRIGATION. 



leverage, and consequent resistance, and also greatly 

 adding to the tendency to resist sliding. The more this 

 weight at the bottom is increased, the stronger, there 

 fore, is the dam. This principle of calculation applied 

 to a bank of earth, or any other construction of the form, 

 shown in fig. 101, will easily show that the power of 

 resistance to overthrow is immensely increased when long 

 slopes are made instead of vertical walls. Besides this 

 increase, the downward pressure of the body of water 

 upon the inner slope, adds to the resistance against both 

 overturn and sliding, and when the foundation is excavat 

 ed, as shown in the illustration, this tendency to resist 

 sliding is again increased, because the adhesion between 

 the old and new earth is rendered more perfect. The 

 thorough incorporation of the old and new surfaces of 

 earth must be carefully made, as a preliminary condition 

 of stability. The full conditions of stability include a 

 weight of bank which with the vertical pressure exercised 

 by the water, to hold it down, will equal the horizontal 

 pressure of the water, against the dam, and leave a sur 

 plus to meet any unexpected contingencies. In addition 

 to these, the materials of the construction must be of 

 such a character as will resist percolation of the water, 

 and will bond together intimately and with cohesion. 

 It is not often that dams give way by sliding upon their 

 foundations ; but an instance of this has happened in the 

 author s experience, when from faulty construction an 

 earth dam, founded upon a smooth rock bottom, gave 

 way bodily to the pressure of the water. But this dam 

 was made by an inexperienced man, in defiance of pro 

 fessional advice, and of proper principles of construction. 

 The best examples of the inside slope of a dam is either 

 3 feet horizontal to 1 foot perpendicular, or S ^to 1. 

 The outside slope may be from I 1 ], to 3, to 1, depending 

 upon the character of the material, and the means used 

 to prevent the surface from washing or crumbling away. 



