PNEUMATICS. 313 



423. The velocity of the wind varies from one 

 that is hardly sensible, to one of 100 miles in an 

 hour. 



The force of the wind, is as the density of the air 

 multiplied into the square of its velocity. SMEA- 

 TON has given a table of its force, expressed by its 

 pressure in avoirdupois pounds, on a plane of one 

 foot square directly opposed to it. Phil. Trans. 

 vol. li. p. 165., &c. Also CAVALLO, vol. ii. p. 287. 



A gentle light breeze is there estimated as moving at 

 the rate of four or five miles an hour, and pressing 

 with a force of about two ounces : A brisk but 

 pleasant gale from ten to fifteen miles ; with a force 

 from half a pound to a pound : A high wind goes 

 at the rate of thirty or thirty-five miles, and pres- 

 ses with the force of five or six pounds : A hurri- 

 cane, that tears up trees, and blows down houses, 

 has a velocity of one hundred miles an hour, 

 and a force of forty-nine pounds on a square 

 foot. 



The velocity of the wind may be estimated by the ve- 

 locity of clouds, or of light bodies carried by it. 



Rain. 



