Page:Popular Science Monthly Volume 58.djvu/220

212 supplied in several experiments in which the man did no considerable external work was 2,500 large calories. The excess in the work experiment was therefore 1,226 calories. Dividing the work done, 256 calories, by the excess of energy absorbed, 1,226, and the quotient is .21. Thus 21 per cent, of this excess of energy absorbed was converted into work, or the efficiency of the man as a machine for doing work is 21 per cent. This is far greater than the efficiency of small portable steam engines, such as could be compared with respect to size or power with a human machine, and equals or surpasses that of the largest compound condensing engines taken in connection with the most perfect water-tube boilers.

The bicycle-dynamo combination is not the most effective device upon which to develop mechanical power; and in the experiments quoted no attempt was made to secure the maximum efficiency of conversion of the potential energy of foodstuffs into mechanical energy. Although many experiments have already been carried out, further experiments are needed to show more fully what the human machine is capable of doing, and what circumstances are favorable to a high efficiency of conversion.

It may be of interest to show how a man's weight varies during twenty-four hours. The accompanying diagrams give the variation in the weight of the man under investigation in one of the rest experiments; that is, in a four-days' experiment, where no mechanical work was done, except that involved in eating, dressing and making some records and observations within the calorimeter. The routine followed each day was nearly but not exactly the same, and the fluctuations of weight are accordingly similar but not identical each day.

Increase of weight is due to food and drink taken into the body and oxygen respired from the atmosphere. Decrease of weight is due to feces and urine leaving the body, and carbon dioxid and water vapor carried away from the lungs and skin. Part of these changes in weight occur more or less suddenly, while the change due to respiration, in which oxygen is absorbed and carbon dioxid and water vapor are evolved, is gradual. In the diagrams the sudden changes are indicated by vertical lines, the numbers indicating the quantity of the change in grams. The gradual