1 93 Dynamics. 



pression for the time employed in passing from A to any point 

 M. We have only to put for * its value c, and for v, its value 

 277. \s %gh , and we have 



Now we have seen how h is determined by experiment, and we 

 know that g = 32,2 feet. 



"When the inclination is 45, cos a being \/? or \/% w have 



Trig.20. 



t = 



hence, if the point M is in a horizontal line, which gives c equal 

 > to the range or to 2 ft, we obtain 



2ft 



This general expression for the time may be made use of in reg- 

 ulating the fusees of bombs. We proceed now to the subject of 

 ricochet firing. 



310. By the above term is meant a motion by which a pro- 

 jectile, after meeting with an obstacle, rebounds and commences 

 a new motion similar to the first. The smaller the angle of ele- 

 vation above the horizon, the greater, other things being the same 

 is the tendency, upon rebounding, to proceed forward ; since the 

 projectile force is exerted almost entirely in a horizontal direc- 

 tion, and much time is required for the resistance of the air and 

 other obstacles to destroy it. If the projectile be of an unelastic 

 substance, and the surface upon which it falls be horizontal and 

 unyielding, it would not bound, since upon arriving at C, accord- 

 'ing to any direction MC, its velocity might be decomposed into 

 two others, of which QC, perpendicular to the surface would be 

 simply destroyed, the rebounding in other cases being caused 

 entirely by the elasticity, so that the other part PC remains (no 

 account being taken of friction and the resistance of the air), and 

 the body would move along CZ. 



