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streams, of uniform depth, in contact with each other, each hav- 

 ing the same tendency, the result must be to produce swelHngs 

 and contractions throughout the whole extent of the dam. Now, 

 when each of these waves strikes the bottom, it gives a blow pro- 

 portioned in force to the body of water falling from the height of 

 the dam. A certain depth of water running over the top of the 

 dam must fall a certain distance before it would be entirely sepa- 

 rated into globules. The smaller the quantity of water, the less 

 distance is required ; consequently every variation in the depth 

 of the water causes a variation in the size and distance of the 

 waves, or tumors ; each of these causes a concussion propor- 

 tionate in intensity to the weight of water in it, and proportionate 

 in rapidity to their distance apart. 



These effects of falling water should be expected, in general, 

 only in artificial falls, such as mill-dams. In natural falls, it is 

 rare that a vertical face is presented for the water to fall over ; 

 and even if such a fall is presented, it is usually formed of angu- 

 lar rocks, causing various depths of water ; and as every varia- 

 tion of depth alters the conditions, the space required to form the 

 tumors in, there would be no coincidence among the tumors 

 formed in different parts of the falling stream ; so that tlie waves 

 of one part would strike the bottom, in the intervals of those of 

 another part, and tlius the concussion of one neutralize the other. 

 Ao-ain, to produce the vibrations, the stream should fall from an 

 equal height throughout its width. If one part falls thirty feet, and 

 another twenty-five feet, the same result is produced as by differ- 

 ent depths of water. For this reason I conclude that vibrations 

 are not conspicuous at Lawrence. There the dam is diagonal to 

 the stream, and there is considerable difference in the height of 

 the fall, at the two ends of the dam. At Hadley the conditions 

 for causing vibrations are, if we cannot say at the maximum, at 

 least the most favorable of any we know. The height of the 

 fall is considerable — great for an artificial work — and so is the 

 width of the stream. The height is uniform from side to side. 

 The dam is one right line from bank to bank, the bed of the 

 river is solid rock, an almost level floor, free from loose or piled 

 up masses. The top of the dam is perfectly level. Conse- 

 quently there is an uniform depth of water passing over the 

 whole length, falling an uniform distance. The waves or tumors 



