92 THE AMERICAN BEAVER. 



rising work. After a quantity of materials had be- 

 come firmly anchored in the bed of the stream, the 

 tendency would be to a downward movement of its 

 margins by the force of the water, which would give 

 to it at its commencement a curvilinear form. With 

 the obstruction of the channel a pond would begin to 

 rise, but the surplus water would pass by on either 

 side at a higher level; consequently, as the work pro- 

 gressed, the contest with the water would be renewed, 

 with similar results at other points, and when the 

 dam was raised sufficiently high, and extended suffi- 

 ciently f\ir to arrest the flow of the water in open 

 channels, and to discharge it through the dam, it 

 would be very sinuous throughout its entire extent. 

 Such, in fact, is the general character of all the dams 

 constructed upon the smaller brooks. In larger 

 streams, with their channels deepest in the centre, 

 we may conceive of a downward movement of their 

 materials by the force of the current, or the pressure 

 of the water at the point where the stream is the 

 deepest, and that this movement may have occurred 

 while the work of construction was in progress. A 

 downward curve is much more common than the 

 reverse in the larger streams. It is not a little sin- 

 gular that the dams across the streams that discharge 

 the largest volume of water are shorter and lower 

 than those upon the smaller brooks, and that in the 

 former the prevailing direction of the curve at the 

 highest point in the structure is down stream, while 

 in the latter it is in the opposite direction. The 

 mode of construction undoubtedly varied with the 

 character of the stream, and with the volume and 

 rapidity of the current. A comparison of a large 



