^rOTION AGAIN-ST THE WIND. 247 



teen square feet. Three times sixteen, or 48 lbs., are con- 

 sequently required to be overcome with every onward 

 step of the horse. Now, we have already seen, when 

 treating of '^ application of labor," that a horse traveling 

 three miles an hour for eight hours will overcome only 83 

 lbs. with ordinary working, which is not double the resist- 

 ance of the wind. Hence we perceive that more than 

 half the horse's strength is lost by driving against such a 

 current. At six miles an hour, all his strength, without 

 over-driving, would be expended in overcoming the force 

 of the wind, and the power required for moving the car- 

 riage would be so much excess of labor. For simplifying 

 the operation, the increased motion of the wind occasion- 

 ed by driving against it has not been taken into account. 

 Even with a small pressure, the loss in power is consid- 

 erable for an entire day. When, for example, the air is 

 perfectly still, traveling six miles an hour will cause a con- 

 stant resistance of 3 lbs. on the carriage, or one-fourteenth 

 of the power exerted for a full clay's work. The same 

 speed against a " gentle wind " of six miles an hour, add- 

 ed, would increase the resistance fourfold, or equal to 12 

 lbs. ; more than one-fourth of the horse's strength at six 

 miles an hour through the day. 



WIND-MILLS. 



The power possessed by the sails of a wind-mill may be 

 nearly ascertained in the same way, the area of the sails 

 being known, and first deducting their average velocity. 



In all wind-mills, it is important that the sails should 

 have the right degree of inclination to the direction of the 

 wind. If they were to remain motionless, the angle 

 would be different from that in practice. They should 

 more nearly face the Avind ; and as the ends of the sails 

 sweep around through a greater distance and faster, they 

 should pres-ent a flatter surface than the parts nearer the 



