Evaporation both by the skin and lungs is reckoned by Rubner at 558 calories during rest; by Helmholtz from the lungs alone it is reckoned at 397.5 calories.

Evaporation from the skin varies according to the secretion of perspiration, and this varies enormously. On an average it certainly does not exceed 700 c.cm. But it can amount to 4 litres in twenty-four hours, and every litre in evaporating at a temperature of 37° C. uses 580 calories. A large part of the expenditure in evaporation is borne certainly by the air, but a considerable part is borne by the organism.

A complete summary of the expenditure and receipts of the organism is worked out on p. 118. I must here omit the experimental technique.

To obtain a correct idea of the development and use of forces in the movements of the body, especially in the most important of all movements from the hygienic and therapeutic point of view, namely, walking, we must remember the most important of the forces which help or hinder our muscular force.

We know that every moment different forces make themselves felt, and that a movement which has once been imparted to a mass according to the laws of physics is continued to all eternity, so long as no other forces counteract and diminish it, or work with and increase it. On the earth we are continuously under the influence of gravity, which to a great degree influences the visible expression of our muscle work and the expenditure on movement. We have moreover the resistance of the medium in which we live.

In comparison with the resistance to our movements offered by the mass of the body, gravity, and the resistance of the air, we may fortunately consider the friction in our joints, tendon sheaths, between muscles, etc., and other slight resistances as "negligible quantities."

We all know that it takes more coal to drive a steamer by water (and air) at 20 knots an hour than to drive it the same distance at 10 knots, even though a speed of 20 knots has once been reached,(Pettcnkofer and Voit).

A man (69.3 kilos in weight; after the experiment G9.5 kilos)

The above table gives the reader an idea of the most important points, in such an investigation, as to our need of water and of the elements C, H, N, and O in their mutual relation, and of salts. But it applies to a man in a condition of comparative rest.

since the resistance of water and air increases with the velocity, neglecting the slight necessary friction between the different parts of the machinery. In walking similar influences are felt, and, in spite of the above-mentioned fact that slow walking may use more muscle work than quick walking, as a rule quick walking uses more muscle force and chemical force (in fat and carbohydrate), due to the resistance of the air, friction in the joints, etc.

Since the expenditure for this resistance is comparatively greater for a small than for a tall person, the latter, especially in quick walking, moves with less expenditure than the former.

Movement through a certain distance on the horizontal costs less than movement for the same distance uphill, more than movement for the same distance downhill; in the former case the movement is hindered, in the latter case helped, by gravity. It costs more to pull oneself up between ropes and overcome the force of gravity by concentric muscle work than to come down the same distance and merely exercise the necessary resistance against gravity to moderate the rate of fall. To walk, and so impart movement to the body mass, costs more than to stand. It costs more to run than to walk, partly because of the more rapid movement and the thereby increased resistance, partly because at each step in running one entirely leaves the ground. It is still harder to jump, since one throws the body in a direction more or less at right angles to the force of gravity.

Marey has shown that those who walk with bent knees and body leaning forward walk farther for a certain number of calories than those who walk with more upright bearing. To keep the knees bent means a greater expenditure than to keep them straight, because the muscle work holding the knees bent at an angle is greater than with straight knees, since the body weight has more leverage. But the lifting of the trunk which is constantly taking place in walking is greater with straight than with bent knees, and it is this that determines the result.

Lastly, we have good reason to believe that a fresh worker who has rested well can perform more external work for the same expenditure than a tired worker or one who is ill, a subject to which we shall return in considering over-strain, and which I consider is not yet fully explained, but which probably is due to the fact that fatigue, illness, or other unfavourable influences make it impossible completely to control innervation of the antagonists of the working muscles.

To calculate the work in hill-climbing one multiplies the height of the mountain by the weight of the climber and adds to the product a value obtained by multiplying the distance travelled by a coefficient which changes and rises with the velocity, also multiplying this product by the body weight. In other words, one adds to the mechanical work of the lifting the mechanical work for the distance taken as if horizontal. Leo Zuntz calculated that for a person weighing 75 kilos to climb a height of 2,200 metres in a distance of 22 km. the expenditure is 75 X 2,200 = 165,000 kilogrammetrcs for the height + 132,000 kgm. for distance = 297,000 kgm.

He further calculated that in walking on the horizontal at a speed of 3/6 km. per hour the work = the body weight multiplied by one-twelfth of the distance; with a speed of 6 km. the multiplier rose to about one-tenth of the distance; when the speed increased to 8.4 km. the multiplier increased to one-sixth of the distance. The consumption of oxygen rose with the different speeds three to four to ten times; the inspired and expired air two to three to six times.