3 



ward both shores. Despite the repeated efforts 

 of the builders to extend it in a straight line to 

 the shore, the flow of the water pushed these out- 

 building ends downward, and when they finally 

 reached the shore this fifty-odd feet of dam with 

 the boulder for a keystone had an arch that was 

 about fifteen feet in advance of the bases. 



Not far from where I lived in the mountains 

 when a boy, the beaver built a dam. This had a 

 slight arch upstream. A few years later the dam 

 was doubled in length by building an extension 

 on the end which bowed downstream. It thus 

 stood a reverse curve. Later the dam was still 

 further lengthened by a comparatively straight 

 stretch on one end, and by a short, down-bowing 

 stretch on the other. Recent additions to this 

 dam consist of wings at the end which sweep up- 

 stream. The dam as it now stands reaches about 

 three fourths of the way around the pond which 

 it forms. 



It is not uncommon for a dam to be planned 



and built with an arch against the current or 



against the water which it afterwards impounds. 



The most interesting dam of this kind that I ever 



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