56 MECHANICAL AID OF THE EIDER. [CHAP. 



own weight back produces an exactly equivalent pressure 

 forward, in all respects the counterpart of your own 

 motion backward, in intensity and duration. It is useless 

 to dwell on this subject, or to adduce the familiar illus- 

 trations which it admits of. It is a simple proposition of 

 mechanical equilibrium, and any one who is conversant 

 with such subjects must assent to it. 

 Mechanical ^he question whether a jockey can mechanically assist 



assistance 



JOC ey ' his horse, does not rest on the same footing. I believe 

 he can, thus. If a man sits astride of a chair, with his 

 feet off the ground, and clasps the chair with his legs, by 

 the muscular exertion of his lower limbs he can jump 

 the chair along. The muscular force is there employed 

 on the foreign fulcrum, the ground, through the medium 

 of the legs of the chair. The muscular action strikes 

 the chair downward and backward, and if the chair was 

 on ice it would recede, so also would the feet of a horse 

 in attempting to strike forward. If the chair was on 

 soft ground, it would sink, so also would a horse, in 

 proportion to the force of the muscular stroke. But if 



