222 PNEUMATICS. 



Miles an Pressure in lbs. on n„„„,i„,i..„ 



hour. a square foot. Description. 



10 .500 



15 1.125 



20 2.000 ; 



25 3.125 : 



30 4.500 ; 



35 6.125 I 



40 8.000 ; 



45 10.125 I 



Pleasant, brisk wind. 

 Very brisk. 

 Strong, high wind. 

 Very high. 



50 12.500 Storm or tempest. 



60 18.000 Great storm. 



80 32.000 Hurricane. 



100 50.000 Tornado, tearing up trees, and sweeping off 



buildings. 



These forces maybe observed at a time when the air 

 is still, by a forward motion equal to that of the wind. 

 Thus walking moderately gives the faint breeze against 

 the face ; riding in a wagon at six miles an hour causes 

 the sensation of a pleasant wind ; the deek of a steam- 

 boat at fifteen miles produces a brisk blow ; while an 

 open rail-car at forty miles an hour occasions a sweep 

 of the air nearly resembling a tempest. 



The preceding table will enable any one to calculate 

 with considerable accuracy the amount of draught 

 which a horse must constantly overcome in traveling 

 with a covered carriage against the wind, adding, of 

 course, the speed of the horse to that of the wind. For 

 example, suppose a horse with a covered carriage is 

 driven against what we term " a very brisk wind," 

 blowing 24 miles an hour, and pressing 3 lbs. on the 

 square foot. The carriage top offers a resisting surface 

 four feet square, or with sixteen square feet. Three 

 times sixteen, or 48 lbs., are consequently required to 

 be overcome with every onward step of the horse. 



