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POPULAR SCIENCE MONTHLY. 



parts with respect to each other. (C) A 

 store of chemical potential energy in 

 its tissues and in food undergoing as- 

 similation. Now when a man walks up 

 hill, A increases, B remains nearly con- 

 stant (increasing slightly), while O de- 

 creases rapidly, due partly to the in- 

 crease of A and partly to the loss of heat 

 by radiation and respiration. When 

 he walks down hill, A is transferred to 

 C or B, or both, and because of this ac- 

 quisition C decreases more slowly than 

 it would do if it received nothing from 

 A, while yet giving off energy at the 

 same rate. The man does positive work 

 upon his body when he lifts it against 

 the force of gravity, storing up poten- 

 tial energy A; he does negative work 

 when he goes down hill, and the energy 

 A passes to the interior of the body. 



Suppose a laborer lifts 20,000 pounds 

 of brick 5 feet; he does 100,000 foot- 

 pounds of work, this energy being trans- 

 ferred from A to the bricks, and it will 

 remain in the bricks as long as they re- 

 main at their elevated position. Next, 

 suppose he lowers the same bricks to 

 their former position. This 100,000 foot- 

 pounds of energy is now transferred 

 back from the bricks to the laborer's 

 body. Because he is expending energy 

 all the time he will possess less energy 

 at the end of the task than at the be- 

 ginning. Nevertheless, he does not lose 

 as much as though he had not received 

 the 100,000 foot-pounds of energy from 

 the bricks, and had given off the same 

 amount of energy in other ways. 



We do not understand the process 

 whereby the body converts chemical po- 

 tential energy of tissue into mechanical 

 energy; that is, we do not understand 

 how the body does work. Still less do 

 we understand how negative work is 

 done; that is, how the body receives 

 energy from without when it lowers a 

 weight or walks down hill. That it 

 does so acquire energy we cannot doubt. 

 But whether it appears at once as heat, 

 or as some other form of energy, and 

 where the energy so received first ap- 

 pears, has not been proved. Neither 

 have experiments been carried out to 



determine the relation between (1) the 

 quantity of negative work done in a 

 given period, (2) the total heat radiated 

 from the body in the same period, (3) 

 the amounts of oxygen absorbed and 

 carbon dioxid respired, and (4) the ex- 

 cess of energy expended over that ex- 

 pended in the same length of time dur- 

 ing rest. Indeed, to repeat the experi- 

 ments already done with the respiration 

 calorimeter balancing the total income 

 and outgo of energy for a given period, 

 with this important difference, that the 

 subject of the experiment was doing 

 negative work (that is, having work 

 done on him by an external agent) 

 would be an extremely interesting and 

 valuable piece of work. 



Consider now what occurs in walking 

 on a level. The foot and leg are lifted, 

 work is done in lifting them, and energy 

 is stored up in them; they are advanced 

 and lowered to the ground, and this 

 stored up mechanical potential energy 

 is then recovered by the system. The 

 center of gravity of the body as a whole 

 is also raised slightly at each step, but 

 the work done in raising it is only 

 equal to the energy yielded by the body 

 when it descends again to the former 

 level. Assuming an absence of friction 

 against the ground and the atmosphere, 

 the total external work done in walking 

 on a level is zero. Force is exerted in 

 holding the body erect or in holding the 

 arm in an extended position. But no 

 work is done in either case, for the force 

 is not exerted through any distance. 

 So also force is exerted by the huge 

 cables which sustain the Brooklyn 

 Bridge against gravity, but no work is 

 done by these cables so long as the 

 bridge is not lifted. Force is exerted by 

 the foundations of a building in resist- 

 ing the attraction of gravitation upon 

 the mass of the superstructure, but no 

 work is done by the foundation in so 

 sustaining the weight. What the inter- 

 nal work of the body may be when 

 muscle is contracted and force exerted 

 without doing external work is another 

 matter. That question is deserving of 

 careful study, and the respiration calori- 



