case of mechanical stability, whether of a house, a tree, a mountain, or a man walking.
There is another term to be considered in this connection which is used by physicists frequently in a special sense, and that is the word system. An open system is one in which the products of a change do not return into themselves, as, to repeat illustrations already given, a waterfall, a fire, or a whirlwind. A closed system, on the other hand, is one in which these products are retained, or at least the internal changes of shape or stress do not travel away; a pendulum might be spoken of as a closed system, because, while gravity causes it to move down to the lowest portion of its arc, the motion thus acquired carries it beyond this point and up the other side, thus converting actual into potential energy, and this alternate conversion and reconversion will go on forever in the absence of friction. Some machines may also be considered under this head, as a stationary steam engine. The apparatus is indeed receiving steam at one end and dispensing mechanical power at the other, but on the average these balance, leaving the machine as the seat of many complicated stresses playing back and forth against each other. In this respect the engine is a closed system.
Now, it may be laid down as a general proposition that self-limited changes occur only in closed systems. Also that any organized structure, and more especially a living animal, may be considered as a closed system; for, though it is true the animal is dependent on food, and is constantly giving out heat and other forms of energy, still, for any moderate period of time these balance each other, while the organism as a whole is dependent for its integrity upon a constant regulation of its internal states through incessant changes, which, moreover, must be self-regulating in character. Life is but a sequence of these delicately adjusted actions and reactions. Physiology is full of instances of this fact. One illustration may suffice. When a muscle is exercised, a portion of it is oxidized or burned. Some of the products of this oxidation are acids, but the vitality of a muscle is diminished by the presence of an acid. The sense of fatigue is the language by which the nerves inform the brain of this muscular state, and the mandate, Let the organ have repose, is only another way of telling the scavengers of the body to go and take that acid away. Here, then, the law is illustrated; the voluntary change in the muscle, represented by its work, sets up a chemical force which limits and finally stops the change by which it was produced.
One machine, more than any other I know of, represents the play of self-limited forces admirably. It is the alternating dynamo for the production of currents of electricity. It has also many of the attributes of an organic being, and I, almost feel like
