Page:The Building News and Engineering Journal, Volume 22, 1872.djvu/145

 Fes. 16, 1872. THE BUILDING NEWS. 129 ee ance to a grainer, who is able to piece out and follow them with the mottler. Hungarian ash is a light wood, of compara- tively recent introduction. It is a beautifully- marked wood, with silvery mottling over a rich undergrain. In colour it is quiet and neutral, in tone not so warm as maple, and is, therefore, a capital wood for contrasting with other woods ; its principal use is for furniture and inlaying in imitation. Messrs. Kershaw and Bellamy have a capital machine for graining this wood. There being very little variety in the figures or marking, this machine is admirably adapted for imitating such woods. Silver-wood is a species of sycamore, of a light silver-greyish white colour, admirably adapted for inlaying, both as to colour and mottle, which latter is a fine silky ripple, somewhat similar to the mottle on the back of a fiddle, but finer in its grain. It is grained on a white ground, using a little ultramarine blue, drop-black, and vandyke brown to grain with. The overgrain may be done with a thin wash of burnt sienna just sufficiently strong to show. Hore-wood is a similar wood to the above, and may be imitated in the same way, but is only suited for use as an inlay. Amboyna is of a foxy red colour, and when it can be used in combination with other woods, is of great value for its colour. It is filled with a mass of small twisted knots, and is not suitable for large surfaces; panels of small cabinets and inlayed work may be done with it to advantage. New Zealand yew is somewhat similar in colour and markings to Amboyna, but not so high in colour. Tulip-wood is a richly-marked wood, and, as its name indicates, is marked in stripes similar to the tulip. It is only useful as an inlay. Purple-wood is of a rich dark purple hue, with but little of its markings visible ; useful as an inlay. Pitch pine we must not describe here, as we shall have to speak fully of its capabilities and use in our paper on ‘Staining and Inlaying.” Bird’s-eye oak is, we believe, an Australian wood; it is filled with a species of knot somewhat similar to the knot in Bird’s-eye maple, and has also asimilar grain to the com- mon oak, but curls round the knots, which gives it a wavy form. There are several other woods, but we have enumerated sufficient to show what a wealth of material, both in colour and form, we have at command (if rightly used), for decorative purposes, and yet how little use is made of our advantages! In our next we purpose describing the best methods of imitating inlayed woods, and various combinations of woods to form harmonious contrasts. —o——— A SHORT THEORY OF THE TRUSS.* HOMOGENEOUS beam, supported at its ends, ' resists the bending tendency of its own and ex- traneous weights by a strain of compression upon its upper fibres and of extension upon the lower. One of the first properties which the mind perceives in studying these internal forces is that the strain upon any fibre is effective in proportion to its dis- tance from the centre, or, in more exact language, to ils distance from a horizontal line through the centre of gravity of the section under consideration. This fact suggests the idea of giving increased effectiveness to the tensile and compression strains by placing the portions exposed to them at a con- siderable distance apart, and confining them immoy- ably, with reference to one another, by ties and pillars or braces. To this combination we apply the term “truss.” To write an exhaustive treatise upon the truss, and one that might, with confidence, be submitted to the criticism of mathematicians, is a task far beyond the aims and abilities of the present writer. Nevertheless, he conceives that a brief exposition of its chief properties by methods suited to the average comprehension will serve a useful purpose. Institute. Let it be required to devise some means of sup- porting the weight W midway between the two fixed points A and B. Fig. 1 represents one of the methods by which it might be accomplished. Erect the pillars AA’ BB’ upon the fixed points ; join their summits by the horizontal pillars or struts C'A', C’B', resting upon a third pillar CC’, and let the weight be supported by two ties CA’, CB’. To prevent any lateral motion of the frame, join CA, CB by ties. In considering this simple frame, there is no diffi- culty in estimating the strains upon each of the The pressure acting vertically at several members. C is composed of theextrancous weight W, together with the weight of the pillar CC’, and half the weight of the struts C’A’, C'B' and ties CA, CB; or, drawing the vertical lines aa’, bb’ through the middle of each panel, the two ties CA’, CB’ sustain the weight lying between these lines. The tensile strain upon each tie is equal to one-half this weight multiplied by the secant of the angle CA’ A. The compressive strain upon each strut C'A' C'B’ is equal to the tensile strain upon the tie multiplied by the sine of the angle CA' A, or equal to the horizontal component of that tension. Each of the pillars AA', BB' sustains one-half the weight W and one-half the weight of the structure, not including its own weight. Each of the fixed points A, B supports half the weight of the frame and its load We. Now conceive each of the points of support in Fig. 2 to be removed a distance AD = BE=CA=CB farther from the centre. Erect the pillars EE’ DD’. Join D'A’, B'E' by struts AD' BE', AD, BE by ties. Suspend additional weights equal to W from A and B. The portion AA’ BB’ with its extraneous weights is now supported in precisely the same manner as the weight at C in the former case. Drawing cc’, dd’ vertically through the centres of the panels AD, BE, the two ties AD’, BE’ sustain the weight between these lines, and each tie is under a tensile strain equal to one-half this weight multi- plied by the secant of CB'B=BE'E. The pressure upon the struts C'A’ C'B! is now increased by that on A'D’ and B'E’ respectively. The tension on CA, CB is equal to the pressure on A'D', B'E’. Repeating the process of removing the supporting points and supplying the additional parts and weights, wefind as before, that each of the ties DF’, EG’, Fig. 3, sustains half the weight lying between their centres, and its tension is equal to this weight
 * By J. P. Frize.u, C.E., in the Journal of the Franklin