Everyone knows what it is to start a piece of work, either intellectual or muscular, feeling stale--or "cold," as an Adirondack guide once put it to me. And everybody knows what it is to warm up to his job. The process of warming up gets particularly striking in the phenomena known as second wind. On usual occasions we make a practice of stopping an occupation as soon as we meet the first effective layer (so to call it) of fatigue. We have then walked, played, or worked enough, so we desist. That amount of fatigue is an efficacious obstruction on this side of which our usual life is cast. But if an unusual necessity forces us to press onward, a surprising thing occurs. The fatigue gets worse up to a certain critical point, when gradually it passes away, and we are fresher than before. We have evidently tapped a level of new energy, masked until then by the fatigue obstacle usually obeyed. There may be layer after layer of this experience. A third and fourth wind may supervene. Mental activity shows the phenomenon as well as physical, and in exceptional cases we may find, beyond the very extremity of fatigue distress, amounts of ease and power that we never dreamed ourselves to own--sources of strength habitually not taxed at all, because habitually we never push through the obstruction, never pass those early critical points. He then states what has come to be called the law of reserve energy. _It is evident that our organism has stored up reserves of energy that are ordinarily not called upon, but that may be called upon; deeper and deeper strata of combustion or explosible material, discontinuously arranged, but ready for use for any one who probes so deep, and repairing themselves by rest as well as do the superficial strata_.