286 FARM MOTORS 



stopped and the flow of steam from the boiler stopped 

 the temperature rises rapidly. 



401. Strength of muscles. All muscles act through 

 very short distances and upon the short end of the levers 

 composing the animal frame. Acting in this way speed 

 and distance are gained with a reduction in the magnitude 

 of the force. A striking example of the strength of a 

 muscle is that of the biceps. This muscle acts upon the 

 forearm, while at a right angle with the upper arm, as 

 a lever of the second class, with a leverage of i to 6. That 

 is, the distance from the point of attachment of the mus- 

 cle to the elbow is but one-sixth of the distance from the 

 hand to the elbow. A man is able to hold within the 

 hand, with the forearm horizontal, as explained, a weight 

 of 50 pounds, necessitating an exertion of a force of 300 

 pounds by the muscle. Attention may also be called to 

 the enormous strength of muscles of a horse as they act 

 over the hock joint while the horse is exerting his maxi- 

 mum effort, in which case the pull of the muscles may 

 amount to several thousand pounds. 



It is because muscles are able to act only through very 

 short distances that it is necessary for them to act upon 

 the short end of the levers in order to secure the proper 

 speed or sufficiently rapid movement. 



402. Animals other than horse and mule used for 

 power. Dogs and sheep are used to a very limited extent 

 in the production of power by means of a tread power 

 similar to the one shown in Fig. 200 for horses. These 

 may be used to furnish power for a churn or some other 

 machine requiring little power. The use of cattle for 

 power and draft has been practically discontinued in 

 America. An ox at work will travel only about two- 

 thirds as fast as a horse. 



403. Capacity. A man working a crank or winch can 



