exacting constraint as the essential condition of this reorganization.

There is a kind of salamander, the axolotl, which lives a tadpole-like youth and never changes to the adult form unless a stress of dry weather annihilates his watery world; but he lives always and reproduces his kind as a tadpole, and a very funny-looking tadpole he is, with his lungs hanging like feathery tassels from the sides of his head. When the aquatic home of the axolotl dries up, he quickly develops a pair of internal lungs, lops off his tassels and embarks on a new mode of life on land. So it is with our young men who are to develop beyond the tadpole stage, they must meet with quick and responsive inward growth that new and increasing "stress of dryness," as many are wont to call our modern age of science and organized industry.

Stress of dryness! Indeed no flow of humor is to be found in the detached impersonalities of the sciences, and if we are to understand the characteristics of physical science we must turn our attention to things which lead inevitably to an exacting and rigid mathematical philosophy. It certainly is presumptive to tell a reader that he must turn his attention to such a thing, but there is no other way; the best we can do is to choose the simplest path. Let us therefore consider the familiar phenomena of motion.

The most prominent aspect of all phenomena is motion. In that realm of nature which is not of man's devising [5] motion is universal. In the other realm of nature, the realm of things devised, motion is no less prominent. Every purpose of our practical life is accomplished by movements of the body and by directed movements of tools and mechanisms, such as the swing of scythe and flail, and the studied movements of planer and lathe from which are evolved the strong-armed steam shovel and the deft-fingered loom.

The laws of motion. Every one has a sense of the absurdity of the idea of reducing the more complicated phenomena of nature to an orderly system of mechanical law. To speak of motion is to call to mind first of all the phenomena that are associated with the excessively complicated, incessantly changing, turbulent and tumbling motion of wind and water. These phenomena have always had the most insistent appeal to us, they have confronted us everywhere and always, and life is an unending contest with their fortuitous diversity, which rises only too often to irresistible sweeps of destruction in fire and flood, and in irresistible crash of collision and collapse where all things mingle in one dread fluid confusion! The laws of motion! Consider the awful complexity of a disastrous tornado or the dreadful confusion of a railway wreck, and understand that what we call the laws of motion, although they have a great deal to do with the ways in which we think, have very little to do with the phenomena of nature. The laws of motion! There is indeed a touch of arrogance in such a phrase with its unwarranted suggestion of completeness and universality, and yet the ideas which constitute the laws of motion have an almost unlimited extent of legitimate range, and these ideas must be possessed with a perfect precision if one is to acquire any solid knowledge whatever of the phenomena of motion. The necessity of precise ideas. Herein lies the impossibility of compromise and the necessity of coercion and constraint; one must think so and so, there is no other way. And yet there is always a conflict in the mind of even the most willing student because of the constraint which precise ideas place upon our vivid and primitively adequate sense of physical things; and this conflict is perennial but it is by no means a one-sided conflict between mere crudity and refinement, for refinement ignores many things. Indeed, precise ideas not only help to form [6] our sense of the world in which we live but they inhibit sense as well, and their rigid and unchallenged rule would be indeed a stress of dryness.

The laws of motion. We return again and yet again to the subject, for one is not to be deterred therefrom by any concession of inadequacy, no, nor by any degree of respect for the vivid youthful sense of those things which to suit our narrow purpose must be stripped completely bare. It is unfortunate, however, that the most familiar type of motion, the flowing of water or the blowing of the wind, is bewilderingly useless as a basis for the establishment of the simple and precise ideas which are called the "laws of motion," and which are the most important of the fundamental principles of physics. These ideas have in fact grown out of the study of the simple phenomena which are associated with the motion of bodies in bulk without perceptible change of form, the motion of rigid bodies, so called.

Before narrowing down the scope of the discussion, however, let us illustrate a very general application of the simplest idea of motion, the idea of velocity. Every one has, no doubt, an idea of what is meant by the velocity of the wind; and a sailor, having what he calls a ten-knot wind, knows that he can manage his boat with a certain spread of canvas and that he can accomplish a certain portion of his voyage in a given time; but an experienced sailor, although he speaks glibly of a ten-knot wind, belies his speech by taking wise precaution against every conceivable emergency. He knows that a ten-knot wind is by no means a sure or a simple thing with its incessant blasts and whirls; and a sensitive anemometer, having more regard for minutiae than any sailor, usually registers in every wind a number of almost complete but excessively irregular stops and starts every minute and variations of direction that sweep around half the horizon!

Wer will was Lebendig's erkennen und beschreiben

Sucht erst den Geist heraus zu treiben.

Goethe

We must evidently direct our attention to something simpler than the wind. Let us, therefore, consider the drawing of a wagon or the propulsion of a boat. It is a familiar experience that effort is required to start a body moving and that continued effort is required to maintain the motion. Certain very simple facts as to the nature and effects of this effort were discovered by Sir Isaac Newton, and on the basis of these facts Newton formulated the laws of motion.

The effort required to start a body or to keep it moving is called force. Thus, if one starts a box sliding along a table one is said to exert a force on the box. The same effect might be accomplished by interposing a stick between the hand and the box, in which case one would exert a force on the stick and the stick in its turn would exert a force on the box. We thus arrive at the notion of force action between inanimate bodies, between the stick and the box in this case, and Newton pointed out that the force action between the two bodies A and B always consists of two equal and opposite forces, that is to say, if body A exerts a force on B, then B exerts an equal and opposite force on A, or, to use Newton's words, action is equal to reaction and in a contrary direction.

In leading up to this statement one might consider the force with which a person pushes on the box and the equal and opposite force with which the box pushes