246 



MACHINERY IN" CONNECTION WITH AIE. 



The following table exhibits the force of wind at different 

 degrees of velocity : 



Description. 

 Hardly percei^tible. 

 Just perceptible. 



Light breeze. 



Gentle, pleasant wind. 



Pleasant, brisk wind. 



Very brisk. 



Strong, high wind. 



Very high. 



Storm or tempest. 

 Great storm. 

 Hurricane. 



Tornado, tearing up trees, and sweeping off 

 buildings. 



These forces may be 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 si.v miles an hour causes the 

 sensation of a pleasant wind ; the deck 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 

 reseml)ling 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 th^t of the wind. For examj^le, 

 suppose a horse with a covered carriage is driven against 

 what we tenn " a very brisk wind," blow^ing 24 miles an 

 hour, and pressing 3 lbs. on the square foot. The carriage 

 top offers a resisting surface four feet square, or Avith six- 



