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Page:Popular Science Monthly Volume 24.djvu/227

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disposed to set up as the skilled engineer of the human machine, and the authoritative exponent of its proper treatment in all its departments, both when it is working rightly as well as when it goes wrong.

A careful study of the qualities and capacities of one's material is the very first thing to be done before determining the wear and tear to which it is to be subjected, or arranging the work it is to do. This is a comparatively easy matter, when an ordinary machine is to be made, however complicated. The iron and the steel of the locomotive can be most accurately tested. Yet all prudent engineers allow an enormous margin for casualties. The actual strain put on is not half of what the machinery could really bear. Who would subject the plates of a boiler to a pressure just up to their bursting-point? Nature in her mechanics usually makes much more allowance than engineers do. The heart of an animal could send five times the amount of blood that it has to propel at twice the rate of the normal blood-current. The arterial pipes that contain and conduct the blood to the extremities are of sufficient thickness and strength to resist five times the pressure put on them day by day. The stomach in a healthy man has usually the power of digesting twice or thrice the amount of food really needed for nourishing the body. Woe betide the diners-out, if it stopped short just at the point when enough for Nature's wants had been digested! This principle of having a reserve of spare power beyond the ordinary daily needs, only to be called into operation on rare and special occasions, is Nature's principle throughout the whole region of life. She scatters seeds by the million where thousands only can grow.

There is a law of Nature, too, that lies at the very root of the principles I am going to advocate to-night. It is this, that every living being has from its birth a limit of growth and development in all directions beyond which it can not possibly go by any amount of forcing. Man can not add one cubit to his stature. The blacksmith's arm can not grow beyond a certain limit. The cricketer's quickness can not be increased beyond this inexorable point. The thinker's effort can not extend further than this fixed limit of brain-power in each man. This limit is fixed at different points in each man in regard to his various powers, but there is a limit beyond which you can not go in any direction in each faculty and organ.

The capacity for being educated or developed in youth, the receptive capacity of each brain, is definitely fixed as to each brain of each young man and woman.

Then the important laws of hereditary transmission of weaknesses and peculiarities and strong points must be studied and kept in mind, so far as we know them, by the educator of youth. To hear some persons talk, you would imagine that every youth and maid had a constitution as free from faults and weak points, and as little liable to go