THEORY OF THEIR CONSTRUCTION. 



215 



length. To resist this, there is nothing but the weight 

 of the wall, and as we have already the length and hight, 

 the thickness only is needed to give the required resist- 

 ance. The rule for finding this, or to be more precise, 

 for finding the required weight of the wall for its stabil- 

 ity, is to multiply together the hight of the wall in feet, 

 by half the thickness, and by 112, the weight in pounds 

 of a cubic foot of masonry, and divide the amount of 

 pressure, previously ascertained, (10,406), by the sum 

 given. In this case we get 4 J | , feet as the required thick- 

 ness of the structure. 



It is evident that this supposed case is one of the weak- 

 est illustrations that could be chosen, because a wall of 



Fig. 103. 



this character is poorly calculated to resist the pressure. 

 But it is a perfectly safe method of calculation, because 

 all the errors are on the right side. If we take off a 

 portion of the upper part of the wall, and place it at the 

 bottom, as shown by the dotted line in the illustration, 

 fig. 102, it is clear that we remove some weight from a 

 point where it is not needed, and put it where it will give 

 much greater resistance, both to oversetting, and dis- 

 placing ; removing the point upon which the wall must 

 turn in case of overthrow, and therefore increasing the 



