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discussing 'The Human Body as an Engine,' I referred to some experiments made at Middletown with the Atwater-Rosa Respiration Calorimeter, in which a man lived several days in each of the experiments in a sealed chamber of about 180 cubic feet capacity, eating, sleeping and working, while under minute observation. The potential energy supplied to the subject of the experiment through the food which he ate was determined by serving him with accurately weighed portions of the various articles of the prescribed diet, and analyzing and burning in a small calorimeter carefully selected samples of the same. The energy yielded by the subject consisted of three portions, all of which were carefully determined. These were: (1) the heat of radiation and respiration which was measured by the calorimeter, (2) mechanical work done within the calorimeter and (3) potential energy carried off in the refuse products of the body. The immediate purpose of the work was to verify experimentally the law of the conservation of energy for the living body; to show that the total energy taken into the body is equal to the sum of all the energy given out by the body during the same period (provided there is no net gain or loss of energy by the body); to show, indeed, that the fundamental law of physics applies to the animal body, as it does to an engine or a dynamo or any other machine or mechanical system. The law has been amply verified for inanimate systems; it seemed desirable to test it for an organic system. The statement was made in the article referred to that "In some cases the man under investigation worked regularly eight hours a day, the work done being measured by apparatus designed for the purpose." Some inquiry having been made as to how this work was measured, and whether it is possible, after all, to do this, the editor has asked me to answer the inquiry through the columns of the

Confusion often arises in considering questions like the present one through inexact ideas concerning force and work. When force is exerted through a finite distance, work is done and energy is transferred from one body to another; and the work done is equal to the energy so transferred. It is also equal to the force exerted in the direction of the motion multiplied by the distance through which the force acts. For example, when a man lifts a stone he exerts a force equal to that of gravity upon the stone through a certain vertical distance; and the work done is equal to the force exerted (that is, to the weight of the stone) multiplied by the height it is lifted. The energy expended by the body is here transferred to the stone in its elevated position. This energy stored up in the stone is called potential energy, and it remains constant in amount so long as the stone remains at the same level. If the stone falls to a lower level its potential energy is reduced, but kinetic energy equal to the decrease of potential energy appears as heat.

If the man lifts the stone one inch the work is only one thirty-sixth part as much as if he lifts it three feet. If he pull on the stone but does not move it, no work is done, in the mechanical sense. Muscle has contracted and work is doubtless done within the body, but