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The adaptation of the various materials furnished by nature, or elaborated from natural products, to the purposes of constructive art, necessarily requires a certain amount of practical and technical knowledge on the part of the constructor; but it should not be imagined that this knowledge is of so special a character, or so intricate in its details, as to limit its attainment to a select few, and it will be shown to be based merely upon careful and persistent observation, whence are derived data upon which, by an ordinary course of reasoning, the technical principles are founded.

In the present treatise I shall carefully avoid clothing the demonstrations and investigations which will occupy our attention in the language of high mathematics, and shall use such expressions as may be familiar to those who have not been specially educated to consider the subjects here dealt with.

It is obvious that the commencement must be made by an inquiry into the nature of the materials presenting themselves to our notice, and therefore we must consider the constitution of solid matter generally, in order to


ascertain by what properties it is rendered available for constructive purposes, and how these properties act in enabling it to resist external agencies tending to its destruction or deterioration. In using the term destruction, I limit its application to the destruction of a certain body, only in so far as it is rendered useless for some specific purpose to which it is applied.

All so-called solid matter really consists of numerous aggregations of very small particles, each such aggregation forming a molecule, and these molecules are of the same materials as tho whole mass. If by mechanical means a solid mass be broken down into small particles, each particle is still of the same nature as the mass of which it formed a part—that is, its chemical composition is not affected.

In the solid mass itself the molecules are not in actual contact with one another; the mass is not really solid, but is full of pores, or interstices; so the particles are sustained at some distance from each other, this distance forming a characteristic of the material—thus lead is a close and cork an open material.

That the molecules of matter are not in contact is evident from the contraction of bodies under reductions of temperature, or from the effects of externally acting mechanical forces.

If a mass of matter at rest be acted upon in such a way as to distort its shape, and when the acting force is removed it resumes that shape, such a body is said to be “elastic," and it is by virtue of its elasticity that it recovers its normal form. If the original shape is exactly resumed, then the elasticity of the material is said to be perfect, and no disarrangement of its constituent molecules has occurred; but if the elasticity is impaired, then a permanent set or distortion has taken place, which may or may not alter the actual strength of the material according to the circumstances under which it has been brought about.

Let us picture to ourselves the condition of the small particles, or molecules, as they exist when aggregated together in a mass of matter at rest. All the particles are standing apart in space, balanced at certain distances from each other by forces which must be of antagonistic natures, such as attraction and repulsion; for, did attractive forces alone act, the molecules would be in contact, while under the sole influence of repellent agencies they would fly asunder and the mass assume the form of highly attenuated gas, expanding and spreading without limit.

It is not my purpose to enter upon any inquiry as to the nature or quality of these forces or their origin; it is sufficient for the present object to know from observation that they exist in various degrees of intensity in all the materials used in construction, and that we can ascertain their relations to external force by experiment, and so arrive at data as to the inherent strengths of different kinds of material.

When a body is at rest, the two forces must just balance each other, and so keep the molecules in equilibrium. These forces are called the Internal Forces.

By the action of any external force upon a body the equilibrium of its molecules is disturbed ; thus, if pressure be brought to bear upon it simply, the internal attractive force is assisted in the direction of the pressure, and the particles approach each other in that direction, partially overcoming the repelling molecular forces, although these again may act laterally, causing the body as it shortens to become wider : on the other hand, a pulling force will aid the repellent forces and lengthen the body, but those repellent forces become weakened in their lateral action, and as the body stretches it becomes narrower. direct forces, producing stress upon material, and into

These are them all other forces must be reduced in order to compare them with the direct resistances of solids to distortion or fracture.

It was discovered a long time ago that the amount of extension or compression a body undergoes—that is, its actual lengthening or shortening-is, if its elasticity be perfect, in direct ratio to the intensity of the force producing it; thus, if 10 tons will lengthen a certain piece of iron by one ten-thousandth part of its original length, then 20 tons will lengthen it by one five-thousandth part,

and so on.


purposes of comparison and calculation it is necessary to have tabular numbers to give the elasticity of the various materials; for this end certain co-efficients, as they are called, have been thus determined :-Let the weight required to produce a certain measurable elongation of any given material be determined, the body experimented upon being of equal dimensions throughout its length, one inch square and perfectly straight, with the weight so adjusted as to act accurately in the direction of its length. Then multiply the weight by the original length of the bar, and divide the product by the extension of the bar under the influence of the weight. The quotient is called the "modulus of elasticity" of the material, and is the weight that would, were such a thing possible, stretch the bar to twice its original length.

The “modulus of elasticity" having been once determined, it is easy to calculate the extension or compression of a body of given length and sectional area under any given stress, as we have only to multiply the length by the strain per sectional square inch, and divide the product by the modulus of elasticity ; as, acting through short distances, the elastic resistance to compression is measured by the same modulus as the resistance to extension.

I will now consider generally the action of external forces that do not lie directly in the line of the internal resisting forces, for it will presently be found that it is with this class of strains we shall principally have to deal in regard to structures of all descriptions.

In Fig. 1 let A B C D be a side view of the layers of molecules forming a beam supported at the points CD; the material being supposed to be unstrained, the mole

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cules will be uniformly arranged as indicated by the dots. Now let a force, W, be brought upon it so that it becomes bent, then it will assume the form shown by the diagram EFGH, where G and I are the points of support, which react upwards with a total force equal to W acting downwards. It is evident that the molecules in the upper layer, EF, are crowded together by the change of form, whilst those in the layer G H are stretched apart; the layers next to these external ones undergoing similar changes in a less degree, until at the layer I K there is neither crowding nor stretching. There will be compressive strain on the concave side then, of the beam and

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