parts with respect to each other. (C) A store of chemical potential energy in its tissues and in food undergoing assimilation. Now when a man walks up hill, A increases, B remains nearly constant (increasing slightly), while O decreases rapidly, due partly to the increase 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 acquisition 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 potential 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 footpounds of work, this energy being transferred from A to the bricks, and it will remain in the bricks as long as they remain at their elevated position. Next, suppose he lowers the same bricks to their former position. This 100,000 footpounds 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 beginning. 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 potential 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 appears, 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 excess of energy expended over that expended in the same length of time during rest. Indeed, to repeat the experiments 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 resisting 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 internal 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-
