PHYSICO DYNAMIC MOTORS.
The animal, including man as such, represents the motive machines to to be considered in this section. Strictly considered, their treatment would naturally include descriptive anatomy and physiology.
By a comparative analysis the analogy between the working animal and the steam-engine becomes apparent. Like the steam-engine, the ani mal must be supplied with water and with substances containing carbon and hydrogen. These substances are furnished to the animal in the nat ural form of grass, grain, and other vegetables, and also in animal foods capable of free assimilation; but to the steam-engine or the caloric motor the potential energy is supplied in the form of wood, coal, or sonic other combination of carbon and hydrogen. In the animal as also in the caloric machine the carbon and the hydrogen arc chemically converted into car bonic acid and aqueous vapor, which, both from the animal and from the machine, are expelled as waste matter. In both motors the elements inci dentally present in the combination of carbon and hydrogen, such as cal cium, phosphorus, iron, sodium, etc., are likewise given off to a greater or less extent in the form of excreta in the animal and of ashes in the machine. In both, heat is developed by this chemical conversion or combustion, and in the animal as in the caloric engine a portion of the heat is wasted by radiation, etc., or by being passed off in the expelled substances, while another portion is utilized for power, or, expressed dif ferently, produces mechanical effect.
From the quantity of carbonic acid and vapor a human being expels, according to physiological observations, and from the mechanical effect he at the same time develops, according to technological observations, there has been calculated the degree of efficiency or the degree of effect which the human animal exhibits as a thermodynamic machine; and the sur prising and interesting result has been obtained that in this respect he fourfold surpasses the steam-engine. This calculation, however, refers only to the degree of effect of the heat developed in the interior of these two motors thus compared, and is hence only scientifically correct. Prac tically, the fact has to be taken into consideration that the carbon and the hydrogen in the soluble form of animal food are more costly—in other words, entail more labor in their production—than is coal or wood, and that the human motor cannot, like the steam-engine, be fed with insol uble fuel, and, moreover, requires food not only for the production of mechanical force, like the steam-engine, but also for nourishment.
A theory regarding the life and the force of animals from the stand point of thermodynamics, considering the anatomical structure of skeleton and muscles on a basis analogous to the theory of the steam-engine, was attempted as far back as 1660 by the Italian physician Borelli. The bold ness of this attempt, arising from the self-confidence which is incident to the youth of mechanical science, stands, however, in the same relation to the success attained as did the efforts of the Greek philosophers to con struct the science of natural objects a priori through speculation, which gave way only to the Galileoic system of research, advancing laboriously and gradually through experiment to knowledge.
The study of the animal as a machine has become entirely empiric, and its theoretical treatment, being practically barren of results, has been almost totally abandoned. It has been observed that the mechanical effect produced by a physiological motor differs very essentially in the perform ance of the following operations—namely, turning a crank, pressing a lever, pulling on a rope, pushing a bar, walking in a tram-wheel, ascend ing a step-wheel, or treading a tread-wheel. It has also been observed that the effect produced by the animal materially depends on the velocity with which it has to turn, push, lift, or tread, and that the number and the duration of the stoppages allowed for recuperation also form an important factor. Generally speaking, the results of these observations demonstrate that the physiological motor works best with a velocity which is half as great as the utmost speed attainable by the animal • without having to overcome resistance, and these results completely har monize with those reached in regard to nearly all other motive machines.