OK STRAIN. 8 1 



tically through its centre of gravity, offers to an applied force 

 through its leverage and tending to change its position de- 

 termines its " stability of position." 



Equilibrium. Forces are said to be in "equilibrium" 

 when they equal or balance each other, each preventing the 

 other from imparting motion to the body ; so also forces, 

 when multiplied by their respective leverages, are said to be in 

 equilibrium when the action which each exerts maintains the 

 body at rest, and it may be observed that the moment of forces 

 about a point may hold each other and establish the equilib- 

 rium of the body even though the forces themselves fail to 

 balance. Two opposing forces, or the moment of these 

 forces, acting at the same time equally upon an unresisting 

 body, neutralize or destroy each other, the body is at rest and 

 equilibrium is said to exist. Should one force, or the moment 

 of that force, exceed the other, equal parts of each force des- 

 troy each other and any excess of the one over the other is 

 termed the "resultant" of the two forces; and the direction 

 of this excess, or the resultant of the two forces, is exerted in 

 a line bisecting the original angle at which the forces met, and 

 the extent of the force exerted by this resultant is the dif- 

 ference between that offered by the two or more original 

 forces, or the moment of those forces. 



Resistance to Overturning. In analyzing the stability of 

 any structure such as a stand-pipe, the effect of the pressure 

 exerted by the wind against the sides of the tank is to cause 

 motion by a sliding, horizontal movement, and to produce 

 overturning about the toe or base. This tendency is resisted 

 by the weight of the tank itself, acting vertically through its 

 centre of gravity and upon the area of its base. The dispo- 

 sition toward moving horizontally upon its base is opposed by 

 the roughness of the parallel faces in contract, as the bottom 

 plates of the tank and the upper face of the foundations, and 

 is found by multiplying the perpendicular pressure by the 

 "coefficient of friction," but as against the action of the 



