DISCUSSION AND CORRESPONDENCE. 



211 



meter might perhaps lend itself to such 

 an inquiry. 



In the experiments referred to, the 

 man under investigation received daily 

 a known quantity of potential energy 

 in the form of food. Part of this was 

 converted into external mechanical en- 

 ergy and was measured; of the remain- 

 der, part appeared as heat and part was 

 carried away in the refuse products of 

 the body. The internal work of the 

 body is ultimately converted into heat, 

 and appears in the total heat of radia- 

 tion and respiration. Thus energy is 

 expended in causing the heart to beat 

 and the blood to circulate and the lungs 

 to expand. This internal work is not 

 stored up, but is transformed into heat 

 and radiated away with that which re- 

 sults directly from combustion. But 

 external work done, like turning a 

 grindstone or sawing wood, is not repre- 

 sented in the heat radiations of the 

 body. 



In order to do the desired amount of 

 work within the calorimeter, the man 

 operated a stationary bicycle, which was 

 geared to a small dynamo. The front 

 wheel of the bicycle was removed, and 

 the rear wheel served as a driving pul- 

 ley for the dynamo. The latter gener- 

 ated a current, the energy of which was 

 measured by an ammeter and a volt- 

 meter. When this current passed out 

 of the calorimeter, its energy was not 

 included in the heat measured by the 

 calorimeter. But in some cases the cur- 

 rent flowed through an incandescent 

 lamp inside the calorimeter. Then the 

 mechanical energy done by the man 

 was all turned to heat within the calori- 

 meter; part of it through friction in 

 the bicycle and dynamo, part through 

 the electric current which flowed 

 through the lamp. The former was 

 measured as accurately as possible by 

 seeing how much energy was required 

 to drive the bicycle when using the 

 dynamo as a motor, supplying current 

 to the latter from a battery and meas- 

 uring the energy so supplied by an 

 ammeter and volt-meter. The quantity 

 of heat resulting from this friction must 



be subtracted from the total heat meas- 

 ured, in order to ascertain the quantity 

 which was given off from the man's 

 body directly as heat. And in those 

 cases where the electric lamp was inside 

 the chamber (and hence the work done 

 by the subject was converted into heat 

 within the chamber) this total amount 

 must be subtracted from the heat meas- 

 ured to give the amount of heat given 

 off as such by the subject of the experi- 

 ment. 



Thus we measure the quantity of ex- 

 ternal work done; but nothing is here 

 learned about the internal work. The 

 latter is converted into heat within the 

 body and, when radiated away, is meas- 

 ured with the rest by the calorimeter. 

 The amount of external work done in 

 driving this bicycle-dynamo combina- 

 tion in one of the experiments (which 

 continued for 96 hours) was equivalent 

 to 256 large calories per day. This was 

 about 40 watts for eight hours, or 

 788,000 foot-pounds, or 394 foot-tons. 

 The total quantity of energy yielded 

 was 3,726 large calories on the average 

 for each of the four days. Since 256 is 

 about 7 per cent, of 3,726, we see that 

 the man converted 7 per cent, of the 

 energy contained in his food into me- 

 chanical energy, 93 per cent, appearing 

 in the heat of radiation and respiration. 

 This gives the man, regarded as a ma- 

 chine for doing mechanical work, a 24- 

 hour efficiency of 7 per cent. During the 

 eight hours in which work was done 

 the total consumption of energy was 

 about 1,850 calories. Dividing the work 

 done by this figure, we have for the me- 

 chanical efficiency during working time, 

 14 per cent. But there is still another 

 way of reckoning this efficiency. Inas- 

 much as a large part of the energy sup- 

 plied to the body would have been re- 

 quired to do internal work and keep the 

 body warm, if no work had been done, 

 we can fairly charge against the work 

 done only the excess of energy supplied 

 during the days when work was done 

 over that required by the same man 

 when no appreciable external work was 

 done. The average quantity of energy 



