STRENGTH OF MATERIALS. 626 STREPSIPTERA. must be less than the elastic limit of the material. It has long been the practice to call the ratio of the ultimate strength to the working stress the factor of safety. The factors of safety commonly employed for diflTerent materials and for differ- ent forms of load are given by Professor Merri- man as follows : MATERIAL For steady stress (buildiugs) For vary- ing stress (Ijriiiges) For slioclcs (marhiu- ery) Timber .. 8 15 6 1 5 10 25 15 6 7 15 35 20 Wrought iron Steel 10 15 Eesilience. One of the most important prop- erties of a material is its capacity to resist the work of external forces, or its resilience. If a bar is placed in a testing machine and pulled by a force gradually increasing from to F, and this pull produces an elongation equal to c, the work done on the bar is the product of the aver- age pull into the elongation, or one-half Fe. Since the internal stresses in the bar resist the work done bj' the external force, they must also per- form an amount of work represented by one-half Fe, and the measure of resilience of a material is the ju'oduct of the average force exerted upon it multiplied by the elongation which this force produces. Elastic resilience is that internal work which has been performed when the internal stress reaches the elastic limit;, ultimate re- silience is that internal work Avhich has been per- formed when the material is replaced. Resilience, like work (see Work), is measured in foot pounds, or, more commonly, in inch pounds. The higher the resilience of a material is the greater is its capacity to resist the work of external forces. Cast iron has very low resilience and wrought iron and- steel have high resilience. Bending Stress. When a beam is loaded it is bent from its original form and takes a curved shape. The fibres on one side of a loaded beam are, therefore, shortened or compressed and the fibres on the opposite side are lengthened or sub- jected to tension. Midway between the two sides where compression ends and tension begins there are fibres which are neither shortened nor length- ened, and this layer or surface of fibres is called the neutral surface. A simple beam is a bar resting upon supports at its ends and is the kind most commonly in use. A cantilever beam is a bar resting on one support at the middle, or if a part of a beam projects out from a wall or beyond a support it is called a cantilever beam. In a simple beam the lower part is under tension and the upper part is under compression; in a cantilever beam the reverse is the case. Beams almost universally fail by tearing apart under the horizontal tensile stress developed by bend- ing. Mathematicians have calculated formulas for determining the resilience of beams to bend- ing, the safe loads to be put on beams, etc., but a consideration of these is a matter of some in- tricacy and the reader is referred to the books mentioned at the close of this article for such information. From these studies, however, the following important laws regarding rectangular beams have been formulated : The strength varies directly as the breadth and directlv as the square of the depth; the strength varies inversely as the length; a beam is twice as strong under a dis- tributed load as under an equal concentrated load. Strength of Columns. A bar under com- pression whose length is greater than about ten times its thickness is called a column. Columns generally fail under stresses produced by com- bined compression and bending. The phenomena are very complex and their investigation is a problem of great intricacy, for whose solution the reader nuist consult special treatises on the subjects. Colunms are of three kinds: Class a includes those with hinged ends; class 6 includes those with one end hinged and one end fixed (the piston rod of a steam engine is a column of this type) ; class c includes those having both ends fi.xed, these being used in bridge and building construction. It has been found by experiment that class c is stronger than class b and that class 6 is stronger than class «. Torsion. Torsion is that kind of stress which occurs when external forces tend to twist a body round an axis. A shaft which transmits power is twisted by the forces applied to the pulleys and thus all its cross-sections are brought into stress. This stress is a kind of shearing, but the forces acting in different parts of a section are not pai'allel. Special formulas have been de- veloped by engineers for calculating strength against torsional stress. Bibliography. Besides the work of Professor ]Ierriman which has been mentioned, the reader may consult to advantage the following works: Johnson, The Materials of Construction (New York, 1897) : Thurston, Materials of Engineer- ing (ib., 1883) ; Unwin. The Testing of Materials of Construction (ib., 1899) ; Mehrtens, Testing of Materials (ib., 1901). STRE'PHON. In Sidney's Arcadia, a shepherd in love with Mania. In literature, a stock name for a lover. STREPSIP'TERA (Neo-Lat., from Gk. cTpt^tai, strepsai, aor. of c'/ihiisiv, strephein, to twist + iTTepdv, ptcron, wing). A group of insects of rather uncertain rank and position, comprising the curious family Stylopidte, and now thought to fall in the Coleoptera. The group consists of a small number of species, very singular in structure and habits, apparently forming a connecting link between Coleoptera and Hymenoptera. The species are all small, and in their larval state live parasitically in the bodies of bees, wasps, and certain Hemiptera, forming the genera Stylops, Colacina, and Xenos. The first infests bees of the genus Andrena. and Xenos occurs in a genus of wasps (Polistes) . Bees that are carrying or have carried Strepsiptera are said to be 'stvlopized,' because the abdomen of the bee is deformed. These parasites do not cause the death of the host. When about ready to transform the lan'a pushes one end of the body out between two of the alidominal segments of its host. The metamorphosis of the female is very slight, while that of the male is complete. The male leads a short and extremely active life of a few hours only, or in some species of a day. The female never changes position and becomes little more than a sac filled with an enormous number of developing young. When the young emerge they are known as 'triungulins.' How the triungulins find their way to the bee-larv.i> is not known. It is supposed by some writers that parthenogenesis occurs with the Strepsiptera,
