PNEUMATICS. 273 



vis viva is therefore an inference from the principle 

 of the virtual velocities. 



Bodies impelled by Currents in the Atmosphere, 

 or by Winds. 



367. The impulse of a stream of air striking 

 with the velocity V 9 on a plane of the area a 2 , in- 

 clined at an angle i to the direction of the stream, 



sin z 2 . 



77i is a quantity to be determined from experiment, 

 and is constant, while the density of the fluid con- 

 tinues the same. 



This proposition coincides with that which is enun- 

 ciated of non-elastic fluids, 299. 



368. The sails of a windmill are so disposed as 

 to turn in a vertical plane round a horizontal axis, 

 they themselves heing all inclined to that plane 

 toward the same side ; when, therefore, the plane 

 in which the sails turn, is placed at right angles 

 to the direction of the wind, the force on each sail 

 is resolved into two, one at right angles to its sur- 

 face, and the other parallel to it. The latter has 

 no effect, hut a part of the former impels the sail, 

 on the side toward which it makes an acute angle 



VOL. I. S with 



