84 THE HORSE IN MOTION. 



of the horse, that renders necessary the great development of the loco- 

 motive organs and columns of support. The power of resistance of 

 these organs must be equal to the attraction of gravity and counteract 

 it, and at the same time be in such excess as will afford .the means of 

 propulsion in a horizontal direction. The influence of gravity is not 

 affected by motion in the body subject to it, at whatever rate it may 

 be moving. It may be projected into the air by a force greater than 

 that of gravity, but it does not escape from it in any degree. The 

 force that projected it was stronger than that of gravity at the time of 

 the impulse ; but the resistance of the air and the constant force of grav- 

 ity would soon bring the motion to an end without the continuation 

 of that projectile force. 



It is the result of this continuation of force in such directions as will 

 resist the attraction of gravity, and overcome resistance to a movement 

 in a horizontal direction, that we call locomotion. 



There is another physical law to the effect that a body put in mo- 

 tion will continue in motion in the given direction until diverted by 

 another force from another direction. The force with which a body 

 moves above the surface of the ground is determined by multiplying 

 its weight by its velocity, and is called its momentum ; therefore the 

 force of gravity, represented by the weight, being constant in the same 

 body, the momentum will be as the velocity. If the body be repre- 

 sented by an iron ball weighing one thousand pounds, moving at the 

 rate of twenty-five miles an hour in a horizontal direction, it will 

 represent the momentum of a horse of equal weight at full speed. 

 To arrest it suddenly would be its destruction. 



To continue its motion without diminution of velocity requires a 

 continuous application of force, and the greater the velocity the greater 

 is the necessity that the trajectory or line of motion should suffer no 

 deflection, for the force necessary to correct it increases with the 

 momentum. 



From what has been said it follows that the only muscular power 

 required to keep a body in motion, at whatever speed, is that which is 

 necessary to resist the attraction of gravity and overcome resistance. 

 It is plain that, in order to maintain a uniform support of gravity, and 



