We have also learned something about the foods which supply this energy; we must now find out how much fuel (in the form of food) it takes to do different amounts of work, just as the owner of an automobile wants to know how much gasoline per mile or per hour is required to run his machine under different conditions. Very careful experiments have been made on many men in different ways to measure their energy output,

Fig. 75. - Respiration calorimeter, open. From the "Journal of Biological Chemistry." the most accurate and interesting being those made in a respiration calorimeter, a device so delicate as to be able to measure the extra heat given off when one changes from lying perfectly quiet to sitting up equally still, thus adding the work of holding the upper part of the body upright. A respiration calorimeter large enough to hold a child is shown in Figs. 75 and 76. You can see that it consists of a chamber

Fig. 76. - Respiration calorimeter, closed. From the "Journal of Bio-logical Chemistry." with thick walls to prevent loss of heat. In Fig. 75 the door is open. When an experiment is going on the door is closed, as in Fig. 76, air being furnished through special tubes. The walls are fitted with delicate thermometers and every device which will help to get the exact amount of heat given off from the body is employed.

Just as it takes more fuel to run a big machine than a little one, so it takes more energy for a large person than a small one; therefore we must know the weight of the one whose food requirements we wish to calculate, as well as the amount of energy required to do different kinds and amounts of work. The following table will help in calculating the approximate fuel requirements of any grown person. The food needs of children and young people under twenty-five will be discussed later.

Approximate Energy Requirements of Average-sized Man

Occupation

Sleeping............

Sitting quietly.........

At light muscular exercise.....

At active muscular exercise ........................... .... At severe muscular exercise.....

Calories per pound per hour

0.4 0.6 1.0 2.0 3.0

Light exercise may be understood to include work equiv-alent to standing and working with the hands, as at a desk in chemistry or cookery; or work involving the feet like walk-ing or running a sewing machine. Many persons, as stu-dents, stenographers, seamstresses, bookkeepers, teachers, and tailors do little or no work heavier than this.

Active exercise involves more muscles, as in bicycling compared with walking, or exercise with dumb-bells as com-pared with typewriting. Carpenters, general houseworkers, and mail carriers do about this grade of work while on duty.

Severe exercise not only involves a good many muscles, but causes enough strain to harden and enlarge them. Bicy-cling up grade, swimming, and other active sports would be included in this kind of exercise. Lumbermen, excavators, and a few others do even heavier work than this.

Knowing the weight of a grown man or woman, and something of the daily occupation, as in the case of a professional man, we can estimate the probable energy requirement somewhat as follows:

Sleeping, 8 hours; 8 X 0.4 Calories = 3.2 Calories per pound. Sitting quietly (at meals, reading, etc.), 8 hours; 8 X 0.6 Calories

= 4.8 per pound. At light muscular exercise (dressing, standing, walking, etc.),

6 X 1.0 Calories = 6.0 Calories per pound At active muscular exercise 2 hours, 2 x 2.0 Calories

= 4 Calories per pound.

Total Calories per pound for 24 hours, 18; 18 X 154 pounds (the weight of the average man) = 2772, or approximately 2680, Calories per day required. Calculate in this way the energy requirement for various grown persons whom you know.