BEAVER DAMS. 91 



outline of the curve is fully revealed. In order to 

 indicate still more completely the crest line of the 

 dam, a ground plan of the entire structure, drawn 

 from actual measurements, is given in the engraving, 

 Plate VIII. 



It is designed to show the crest line and the lower 

 face of the dam. With the engravings, and the meas- 

 urements in detail, hereafter given, the general ap- 

 pearance, form, and structure of the dam will be fully 

 understood. 



The curve is one of the striking features of a beaver 

 dam. They are almost invariably found where the 

 thread of the stream originally ran, and are restricted 

 to the class of dams now under consideration. In the 

 largest structures, the convexity of the curve is usually 

 up stream, but this is not always the case. Several of 

 those represented on the mnp curved down stream at 

 the point where the dam was the highest. This one 

 shows a reverse curve down stream nearly as large 

 and well defined as the principal one in the opposite 

 direction. It is generally asserted that the introduc- 

 tion of a curve, with its convexity up stream, was the 

 result of intelligence and design on the part of the 

 architects; and that its use at the precise point where 

 the pressure of the water is the greatest, aftords con- 

 clusive evidence that the beavers understood its me- 

 chanical advantages. Whether these curves were 

 the result of accident or of design is a question. We 

 must suppose that this dam was commenced at the 

 thread of the stream where the great curve is found, 

 and it seems not improbable that its curvature may 

 be due to the flow of the water on either side when 

 the original channel was first obstructed by their 



