XLVI. T. W. SEAVER. 



1. That the horizontal thrust of the water, must be held 

 back by the resistance of the masonry to sliding forward 

 or overturning. 



2. That the pressure sustained by the masonry or the 

 foundations must never exceed a certain fixed limit, usually 

 from about 140 to 200 fbs. per square inch. 



3. That by causing the resultant of the forces to fall 

 within the middle third of the base, there shall be no tension 

 in any part of the structure. 



In a new theory of dam strains recently formulated by 

 Mr. Atcherley and Professor Pearson, both of University 

 College, London, they join issue in many of these points. 

 They say that neither the foundation or the dam can be 

 considered absolutely rigid, that other forces are at work 

 in the body of the dam and in the foundation owing to the 

 elasticity of the materials and other causes besides those 

 of water pressure and weight, that there is tension in the 

 front of the dam, even though the resultant falls within 

 the middle third, and lastly that the stresses in the vertical 

 sections are more critical for stability than those in the 

 horizontal sections. They maintain "that the current 

 treatment of dams is fallacious, for it entirely screens the 

 real source of weakness, viz. in the first place the tension, 

 and in the second place the substantial shear in the vertical 

 sections." 



This theory being so much opposed to all former 

 practice, might be considered as out of the range of prac- 

 tical constructive engineering, were it not for the effect it 

 had in postponing the raising of the Assouan Dam. 



Sir Benjamin Baker in his report on this subject, writes 

 thus : — " I have arrived at the definite conclusion that still 

 further experience of the working of the dam is- required, 

 before any responsible engineer, knowing the recent 



