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STRENGTH OF MATERIALS In the use of materials for a structure or a machine it is all-important to the designer to know how much strain a given force will produce, whether the material will recover from the strain when the straining-force is removed, and how great the strain can be before breaking takes place. A structure must be designed so as to withstand without permanent set the loads that it is to carry. In most cases it is equally important to determine what is the least amount of material which can be safely used under the given conditions. Experiments to determine these facts cannot be made on the completed structure, for, while tests of complete structures are important, they cannot reveal all that is necessary to know and, besides, would be an expensive way of determining safety-limits. But by analysis of the forces and loads the designer can, even in complex structures, as bridges and roofs, reduce the forces to comparatively simple straining actions in the individual parts, so that he can make his plans from the results of experiments on the materials used. Such experiments are called tests of strength of materials, and the machines for making the tests are called testing-machines. (See .) The behavior of materials under loads is that, under any load, there is a strain (elongation, compression, bending, twisting etc.), and there is developed at the same time a stress-resisting force which tends to restore the material to its original form. In 1676 Robert Hooke first stated his law that in elastic materials the stress is proportional to the strain, a law which is assumed to be strictly true for structural testing. The ratio between the stress and the strain is called the coefficient of elasticity. For practical purposes there are as many kinds of coefficients as there are kinds of strain, as coefficient of tension, of compression, of bending or of torsion. As long as Hooke's law holds, the material regains its original form as soon as the straining-force is removed. But if the strain goes beyond a certain amount, depending on the material, the material remains permanently deformed or set, and we say it has been strained beyond “the elastic limit.” In engineering-work it is very important that no part of a structure or machine shall be strained beyond the elastic limit. If the load be still further increased, the material will break under the load, and we have the breaking-force or “ultimate strength of the material.” From the above it is seen that the important tests which are commonly required are (1) amount of elongation, of compression, of bending and of torsion produced by different forces; (2) elastic limit in each case; and (3) the ultimate strength or breaking-force in each case. Stresses and straining-forces are, in British and American engineering practice, stated in pounds per square inch. We can

thus express the average tensile strength of timber as 10,000, of cast iron as 20,000, of wrought iron as 60,000 and of steel as 100,000 or over, meaning so many pounds per square inch cross-section. The strength of any material depends not only on the composition of the material but on its physical condition, the treatment it has undergone, the way the load has been applied, and in every case a wide margin must be allowed between the possible loads as shown by the tests and the actual loads and for conditions which cannot be fully taken into account. The term “factor of safety” is used to express this relation between working loads and theoretically possible loads. Thus for a working tensile strain, one fifth of the ultimate strength may be used for good wrought iron or the “factor of safety” is five. Besides the tension, compression, bending and torsion tests, there are a number of other tests which are made on materials for special purposes, as tests on paving-bricks for hardness and durability, tests of cements and limes. For detailed information on the strength of materials and testing consult Unwin's Materials of Construction or Merriman's Mechanics of Materials.  Strikes. See, and.  Strobilus (in plants), a name given to the cone-like cluster of sporophylls which is best displayed by equisetums, club-mosses and gymnosperms. In gymnosperms it often is called a flower, and it really stands for the flower in plants without floral leaves. It probably is the structure from which true flowers have been derived.  Strych′nine. See.  Stu′art or Stew′art, the name of a royal family of Scotland and England. The name is derived from the office of steward of Scotland, which was given to one of the family by David I, king of Scotland, and became hereditary in the family. The family is of Norman origin, the first ancestor in England receiving lands from Henry I. The marriage of Walter, the sixth steward of Scotland, in 1315 to Marjory, daughter of Robert Bruce, brought the crown of Scotland to the Stuarts. “It came with ane lass,” said James V. Robert II, the son of Walter Stewart and Marjory Bruce, came to the throne in 1317 as the first of the Stuart kings. Between 1371 and 1714 14 Stuarts sat upon the Scottish, and six of these also on the English, throne. The last Stuart sovereign was Queen Anne. The house of Hanover, to which the present ruling family of England belongs, is connected with the Stuart family through Sophia, Electress of Hanover, who was the granddaughter of James I. See The Royal House of Stuart by Gibb and Skelton.  Stuart, Gilbert Charles, an American painter, was born at Narragansett, R. I., in 1755. He studied at Edinburgh, worked his