SPEED OF STORM -WINDS. 233 



This force of storms is accounted for by the vast 

 swiftness of their progress. It has been found, by 

 comparing different observations, that hurricanes 

 march on sometimes at the rate of from forty to forty- 

 eight miles an hour, that is, of from sixty to eighty 

 feet in a second. The swiftness of their whirling mo- 

 tion must, however, be far greater. We have not as 

 yet found any sure means of measuring this rate, 

 but it cannot be doubted that it amounts some- 

 times to a hundred and fifty feet in a second. But 

 so swift a motion of the air as this, when it meets 

 with any obstacle, exerts upon it a pressure of about 

 thirty-two pounds on the square foot. The force 

 with which the wind strikes bodies that meet it 

 with their full front, may be easily reckoned from 

 this. The resistances at the surface of the earth, 

 while they receive the pressure of the wind, gra- 

 dually lessen its speed. At great heights, where 

 it meets with none of these checks, for instance 

 on the peaks of lofty mountains, we almost always 

 find the force of the wind far greater than below. 



The air above, therefore, is always outrunning 

 that below it in its onward progress, and thus it 

 happens that changes in the direction of the wind, 

 and the commencement of storms are often felt on 

 high mountains several hours, and even days 

 before they reach the lower ground. It is com- 

 monly remarked, in countries where in winter the 



