208 



POPULAR SCIENCE MONTHLY. 



DISCUSSION AND CORRESPONDENCE. 



ENERGY AND WORK OF THE 

 HUMAN BODY. 

 In discussing 'The Human Body as 

 an Engine,'* I referred to some experi- 

 ments 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 capa- 

 city, eating, sleeping and working, 

 while under minute observation. The 

 potential energy supplied to the sub- 

 ject 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 sam- 

 ples of the same. The energy yielded 

 by the subject consisted of three por- 

 tions, all of which were carefully deter- 

 mined. These were: (1) the heat of 

 radiation and respiration which was 

 measured by the calorimeter, (2) me- 

 chanical work done within the calori- 

 meter 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 funda- 

 mental law of physics applies to the 

 animal body, as it does to an engine or 

 a dynamo or any other machine or me- 

 chanical system. The law has been 

 amply verified for inanimate systems; 

 it seemed desirable to test it for an 

 organic system. The statement was 



* Popular Science Monthly for September, 



1900. 



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 ap- 

 paratus 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 in- 

 quiry through the columns of the 

 Monthly. 



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 en- 

 ergy 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 ex- 

 ample, when a man lifts a stone he ex- 

 erts a force equal to that of gravity 

 upon the stone through a certain verti- 

 cal 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 ex- 

 pended 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 



