Page:Popular Science Monthly Volume 46.djvu/519

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will necessarily be somewhat materialistic in terminology, because we are dealing with material things and material forces, the writer does not wish his words to seem to be conditioned by any metaphysical system.

Practically speaking, we are dealing with concrete things—stones, bricks, men and women, society. The materialist says they evolved themselves; the idealist says they were made by an outside agent; but for the purposes of this paper it is all one and the same, because any structure, whether self-created or manufactured by intelligence, is largely conditioned by the substance out of which it is made.

Now, how is society conditioned by that out of which it is made? How do the general properties of matter enter into the natural history of its development? That, I think, is a very interesting and a vitally important question if it could be answered in its entirety, but no one can do this for us yet. All we can do is to pick out a few characteristics so obvious that perhaps they will seem only trite. But granting they are so, still, it sometimes happens that the familiar and the commonplace take on new features when looked at from fresh standpoints.

The first thing, then, to be noted in regard to circumscribing conditions is that the inherent strength of materials puts a limit to the possible size of any structure, whether artificial or natural. As an example, consider a cannon: The actual size of a big gun is not limited by its weight, for much heavier ordnance than any now made could be handled by modern machinery; what stands in the way is the tensile strength of the metal employed. As soon as the pressure of the exploded powder upon a square inch of the internal surface of the gun is greater than the elastic limit of the steel, the metal will give way by stretching. This will enlarge the surface of the chamber, which will thus offer fresh portions for the action of the pressure, and so the operation will go on until rupture takes place. There is thus a certain powder pressure beyond which no thickness of metal, however great, can prevent bursting; this limiting size is already nearly reached in recently built cannon, and nothing but the discovery of some new metal or alloy stronger than steel will enable us to build guns materially larger than those now made.

Another instance is found in bridge work. As the spans grow longer, the proportionate load they can carry grows smaller. It is easy to calculate from the known properties of iron just how long a span must be to barely sustain itself. Nothing longer than this could stand, because, when the weight of the bridge puts a stress on its members greater per square inch than the breaking stress per inch, the bridge must fall, even without any extraneous load. In living creatures the same condition is found. No land animal