14 



Scientific Proceedings, Royal Dublin Society. 



In the case above considered, although as -4 C varies P con- 

 tinues proportional to the square on DE, it is not proportional to 

 the square on AC, so that the pressure on the portion of the wall 

 measured \)j AC does not follow the law of fluid pressure, amd 

 the determination of the point where P should he applied presents 

 considerable difficulty. One subordinate case will be considered 

 further on. 



When the surcharge of earth vanishes, the trapezium AZBC 

 becomes the triangle ABC (iig. 4). In this case the pressure 



Fig. 4. 



on the portion of the wall measured by AC will be obtained by 

 drawing a system of lines parallel to those corresponding to the 

 line -4(7; for drawing such, we have 



AC'D'E' CD"" AC AD' 



or 



or 



again, 



P' 

 P 



DE' 



CD' CA 



%} 



