"Gradual drying and seasoning in this manner is considered the most favorable to the durability and strength of timber. "Timber of large dimensions is improved by immersion in water for some weeks. Oak timber loses about one fifth of its weight in seasoning, and about one third of its weight in becoming dry." 227. Decay of Timber.-There are three principal causes of decay of timber-dry-rot, wet-rot, and the "teredo navalis" and other worms. Dry-rot does not usually occur where there is a free circulation of air, and if the timber is properly dried an occasional immersion in water should do no harm. Timber kept dry and well ventilated has been known to last for several hundred years without apparent deterioration. Dry-rot is caused by a species of wood fungus-Merulius lachrymans—which destroys the tensile and cohesive strength, gradually converting the timber into a fine powder. Wet-rot. This is the destructive agent at work more or less on all timber freely exposed to air and moisture. It is of two kinds : A. Chemical.-In this case a slow combustion takes place, and by a gradual process of oxidation the wood slowly rots away. B. Mechanical.-This is the more common form, and generally occurs near the water-line in timber subject to frequent immersion. It is the frequent alternate conditions of moisture and dryness that are most trying to timber, as is the case with metals. When timber is constantly under water, the action of the water dissolves a portion of its substance, which is made apparent by its becoming covered with a coating of slime, and this protects the interior. If, however, it is exposed to alternations of moisture and dryness, as is the case with piles in tidal waters, the dissolved parts being continually removed by evaporation and the action of the water, new surfaces are being frequently exposed for decomposition. Piles driven in sea-water are frequently destroyed by the "teredo navalis," and also by another species of worm called the "limnoria." They both work from about the high-water mark to the surface of the mud. 228. To test Steel and Iron. Scientific American.Nitric acid will produce a black spot on steel; the darker the spot the harder the steel. Iron, on the contrary, remains bright if touched with nitric acid. Good steel in its soft state has a curved fracture and a uniform gray lustre; in its hard state, a dull, silvery, uniform white. Cracks, threads, or sparkling particles denote bad quality. Good steel will not bear a white heat without falling to pieces, and will crumble under the hammer at a bright-red heat, while at a middling heat it may be drawn out under the hammer to a fine point. Care should be taken that before attempting to draw it out to a point the fracture is not concave; and should it be so, the end should be filed to an obtuse point before operating. Steel should be drawn out to a fine point and plunged into cold water; the fractured point should scratch glass. To test its toughness, place a fragment on a block of cast-iron: if good, it may be driven by a blow of a hammer into the cast-iron; if poor, it will crush under the blow. Tests of Iron.-A soft tough iron, if broken gradually, gives long silky fibres of leaden-gray hue, which twist together and cohere before breaking. A medium even grain with fibres denotes good iron. Badly refined iron gives a short blackish fibre on fracture. A very fine grain denotes hard steely iron, likely to be coldshort and hard. Coarse grain with bright crystallized fracture or discolored spots denotes cold-short, brittle iron, which works easily when heated and welds well. Cracks on the edge of a bar are indications of hot-short iron. Good iron is readily heated, is soft under the hammer, and throws out few sparks. 229. Strength of Rope.-The table on following page gives some idea of the strength of ordinary Manilla Rope. It must be remembered that these values are for new ropes and that a few months' exposure to the weather will probably cause a decrease in the strength of 40 or 50 p. c. A factor of safety of 4 or 5 is generally employed to obtain their safe working strength. Ropes made of good Italian hemp are considerably stronger than these. Wire Ropes.-The following table gives the strength of iron and cast-steel wire rope : TABLE OF IRON AND CAST-STEEL WIRE ROPE. These ropes have 19 wires to the strand and hemp centres. One fifth of the above breaking-strength may be taken as the safe working strength. For the strength of Iron Rods see Sec. 138. 230. Properties of the Circle. Length of arc = number of degrees X 0.017453 radius. Arc of 1° to rad. 1 0.01745329. Arc of 1' to rad. 1 = 0.000290888. Arc of 1' to rad. 1 Degrees in arc whose length 0.000004848. radius = 57°.2957795. π = 3.1415926536; Log = 0.4971499. angle GAE: = unity, H 231. PLANE TRIGONOMETRY.-In Fig. 96, if the 90°; then in the rightangled triangle ABC, if AB = Radius = G F D B An angle and its Supplement have the same Sine and Cosecant; but the Tangents, Secants, Cosines and Cotangents, though of equal length, are of contrary signs: so that in applying to obtuse angles trigonometrical formulæ which were originally intended for acute angles, the algebraic signs of the tangents, secants, cosines, and cotangents must be reversed. The sine, secant, and tangent of an angle A are respectively equal to the cosine, cosecant, and cotangent of its complement (i.e., of 90° — A). Examples of Right-angled Triangles: |
