Are you an unusually persevering and persistent person? Or, like most of us, do you sometimes find it difficult to stick to the job until it is done? What is your usual experience in this respect?Is it not this, that you work steadily along until of a sudden you become conscious of a feeling of weariness, crying "Enough!" for the time being, and that you then yield to the impulse to stop?Assuming that this is what generally happens, does this feeling of fatigue, this impulse to rest, mean that your mental energy is exhausted?Suppose that by a determined effort of the will you force your lagging brain to take up the thread of work. There will invariably come a new supply of energy, a "second wind," enabling you to forge ahead with a freshness and vigor that is surprising after the previous lassitude.Nor is this all. The same process may be repeated a second time and a third time, each new effort of the will being followed by a renewal of energy.Many a man will tell you that he does his best work in the wee watches of the morning, after tedious hours of persevering but fruitless effort. Instead of being exhausted by its long hours of persistent endeavor, the mind seems now to rise to the acme of its power, to achieve its supreme accomplishments. Difficulties melt into thin air, profound problems find easy solution. Flights of genius manifest themselves. Yet long before midnight such a one had perhaps felt himself yield to fatigue and had tied a wet towel around his head or had taken stimulants to keep himself awake.The existence of this reserve supply of energy is manifested in physical as well as mental effort.Men who work with their heads and men who work with their hands, scholars and Marathon runners, must alike testify to the existence of reserve supplies of power not ordinarily drawn upon.