The force of the wind increases directly as the square 

 of the velocity. Thus, a wind blowing 10 miles an hour 

 exerts a pressure four times as great as at 5 miles an hour, 

 and 25 times as great as at 2 miles an hour. 



To find the force of wind acting directly against a sur- 

 face. 



EULE. Multiply the surface in square feet by the Ibs. 

 pressure per square foot as given in the following table. 



EXAMPLE. What is the pressure of a wind of a velocity 

 of 20 miles per hour against a barn door 10 feet by 6 ? 



SOLUTION. 10x6=60 sq. ft., surface, x2 Ibs., pressure 

 per square foot, = 120 Ibs. Ans. 



