68 



THE ANIMAL AS A MACHINE. 



cation of force in Fig. 2, is now much below the hori- 

 zontal line MN^ drawn through the point C on the 

 resultant, or virtual line of draught. Comparing the 

 triangles ABE (Fig. i) and PAO (Fig. 2), AB and 

 PA represent the pull through the traces, and, al- 

 though the force is the same — 36 lbs., the result is 

 different. 



If AB in Fig. i represents a 36-lb. pull, and AC 2. 

 lo-lb. Hft, then AD (33 lbs.) is the resultant direction 

 of force applied by the animal. A 36-lb. pull through 

 AB, together with a lift of 10 lbs. through AC ox 2. 

 pull of 33 lbs. through AD, will both be effective in 

 lifting 2.6 lbs. from the horse's fore quarters.* 



Thus the animal may be very effectively aided, or 

 may be totally incapacitated by good or bad adjust- 

 ment of the line of traction. In all cases this line 

 should be given such inclination as will insure increased 

 pressure of the animal upon the ground for heavy pull- 

 ing, and decrease its weight in the opposite case, just 

 to the extent required to make adhesion ample with- 

 out unnecessary surplus. 



21. Muscular Power varies enormously with the 

 animal, its condition, habits of exercise, and other cir- 

 cumstances ; but the results of many experiments indi- 

 cate that the muscles of the human body have a power 

 measured, in good condition, by a maximum of not 

 far from 10 kilogrammes per square centimetre, and 

 averaging 7 or 8 (142,1 10,1 14 pounds per square inch 

 respectively). The influence of exercise and custom 

 on the working power of the muscle is enormous, in 

 some cases being found to vary in the proportion of 



* Ibid. 



