82 



TOWERS AND TANKS FOR WATER-WORKS. 



wind upon the sides of a stand-pipe, the vertical pressure ex- 

 erted even by the weight of the empty tank over the area of 

 its base, is usually sufficient to restrain the force exerted by 

 the wind and to keep the structure at rest even without the 

 customary anchorage, therefore this tendency will not be 

 given further consideration here. 



The effect which wind exerts upon cylindrical structures 

 such as a stand-pipe has never been determined with any de- 

 gree of certainty, but Trautwine has the following: 



Wind Pressure. " The relation between the velocity of 

 wind and its pressure against an obstacle placed either at 

 right angles to its course, or inclined to it, has not been well 

 determined, and still less so its pressure against curved sur- 

 faces. The pressure against a large surface is probably pro- 

 portionately greater than against a small one. It is generally 

 observed to vary nearly as the square of the velocities, and 

 when the obstacle is at right angles to its direction, the pres- 

 sure in pounds per square foot of exposed surface is considered 

 to be equal to the square of the velocity in miles per hour, 

 divided by 200. On this basis, which is probably quite defec- 

 tive, the following table, as given by Smeaton, is prepared : " 



