The most important modification to be made in the above estimate arises from the differences of work demanded. Men may exist in inaction on a scale of food-supply which is followed by death from starvation when they are put to hard labor. It is of importance, therefore, to have some measure of the effects of physical exertion. And here mechanical science has contributed to physiology a precision rarely attainable in our dealings with social economy. Mr. Joule, of Manchester, analyzed, about thirty years ago, the relation which the heat, used as a source of power in machinery, bore to the force of motion thus made active. He showed that raising the temperature of 1 pound of water 1° Fahr. was equivalent to raising 772 pounds to the height of 1 foot; and conversely, that the fall of 772 pounds might be so applied as to heat 1 pound of water 1° Fahr. Thus, the mechanical work represented in lifting 772 pounds 1 foot, or 1 pound 772 feet, forms the "dynamic equivalent," the measure of the possible strength of 1° of temperature as marked by the thermometer in 1 pound of water. Physiologists seized eagerly on the opportunity which Joule's demonstration seemed to afford them of estimating in actual numerals the relation of living bodies to the work they have to do. So much earth raised on an embankment represents so much heat developed in the machinery, be it living or dead. The fully digested food, converted through several stages into gaseous, liquid, and solid excretory matters, produces by its chemical changes a definite amount of heat, of which a definite amount escapes, and a definite amount is employed in working the involuntary machinery of the body, and the rest is available for conversion at will into voluntary muscular actions.

It may be reckoned that the daily expenditure of force in working the machinery of the body-in raising the diaphragm about 15 times and contracting the heart about 60 times a minute, in continuously rolling the wave of the intestinal canal, and in various other involuntary movements, without anything to be fairly called work-it may be reckoned that the expenditure of force in doing this is equal to that which would raise a man of 10 stone 10,000 feet.

There are several reasons for believing that, in assigning their physiological functions to the several sorts of food, nearly all the business of begetting force should be ascribed to the solid hydrocarbons, starch and oil, by their conversion into carbonic acid and water, just as there are good grounds for thinking that it is the conversion of the solid hydrocarbon of coal into the same substances which drives a locomotive. To the nitrogenous aliments seems allotted primarily the task of continuously replacing the wear and tear of the nitrogenous tissues, while any excess of them assists the starch and oil in keeping up the animal heat.

One of the most cogent of the reasons for this view is that the chief nitrogenous excretion, the urea, is not increased in amount in proportion to the work done, as shown by the experiments of Messrs. Fink and Wiscelenus; whereas the excretion of carbonic acid in a decided manner follows the amount of muscular exertion. Now, it is very clear that, if the supply of power to do work depended on the decomposition and renewal of the muscles by flesh food, the urea must be exactly proportioned to the exertion, which is not the case.

To give an example of the mode of working out a problem by this theory: Prof. Frankland, in a series of experiments made in 1866 at the Royal Institution, and published in the London Philosophical Magazine, vol. xxxii., p. 182, ascertains with the "calorimeter" (which reckons the amount of heat evolved as a thermometer does its degree) the quantity of energy or force evolved under the form of heat during the oxidation of a given weight of alimentary substance. It has been explained that heat and mechanical work, being convertible into one another, bear a constant proportion to one another; so that a definite production of so much heat invariably represents the potentiality of so much motion, used or wasted according to circumstances. From the reading of the calorimeter, therefore, may be calculated how many extra pounds ought to be raised a foot high by a man who has eaten an extra pound of the food in question; how many steps a foot high he ought to raise a weight of ten stone (say himself) before he has worked out the value of his victuals. Prof. Frankland has thus estimated the comparative value of foods as bases of muscular exertion, and he has made out a table of the weight and cost of various articles that would require to be consumed daily to enable a man to support life, the equivalent of which has been already reckoned as the muscular force in action which would raise a man of 10 stone 10,000 feet.

After the supply of sufficient albuminoid matters in the food, to provide for the necessary renewal of the tissues, the best materials for the production of internal and external work are non-nitrogenous matters, such as oil, fat, sugar, starch, gum, etc. "When the work is increased, not so much extra meat as vegetable food, or its dietetic equivalent, fat, is demanded.

In comparing the cost of a daily sufficiency of the various foods to produce the required force, we must not forget the inconveniences which many of them entail. These inconveniences must be added to the cost. For example, suppose a man to have been living upon potatoes only, just supporting life with 5 pounds a day, and then to get work which enabled him and required him to take a double supply of non-nitrogenous food, he would act unwisely if he were to swallow it in the form of 12 pounds of cabbage. He would be knocked up by the sheer labor of carrying 12 pounds extra in a vessel so ill-adapted to.sustain heavy loads as the stomach. A similar objection would lie against milk or veal or apples, however cheap accident might make them; and a more serious objection still would hold against nine bottles of ale, or seven of stout. On the other hand, the over-concentration of cheese, beef dripping, and lump sugar, makes them nauseous when in large quantity or monotonously persisted in, though when introduced as a variety they are appetizing and digestible. There is no saving in using that against which the stomach is set, or which the absorbents refuse to assimilate.