The imperfection of this method is so obvious, that it has seldom been practised : but it has been combined with the other, as is represented in fig. 22. where the beams are divided along the middle, and the tables in each half are alternate, and alternate also with the tables of the other half. Thus 1, 8,4, are prominent, and 5, 2, 6, are depressed. Thii construction evident ly puts a stop to both slides, and obliges every part of both pieces to move together. a 6 and c d show built-up beam corresponding to AB No more is intended in this practice by any in telligent artist, than the causing the twd pieces to act together in all their pans, although the strains may be unequally distributed on them. Thus, in a built-up girder, the binding joists are frequently mortised into very different parts of the two sides. But many seem to aim at making the beam stronger than if it were of one piece ; and this inconsiderate project has given rise to many whimsical modes of tabling and scarfing, which we need not regard.
The practice in the British dock-yards is some what different from any of these methods. The pieces are tabled as in fig. 22. but the tables are not thin parallelopipeds; but thin prisms. The two out ward joints or visible seams are straight lines, and the table No. 1. rises gradually to its greatest thick ness in the axis. In like manner, the hollow 5 for receiving the opposite table, sinks gradually from the edge to its greatest depth in the axis. Fig. 23. No. 1. represents a section of a round piece of timber built up in this way, where the full line EFGH is the section corresponding to AB of fig. 22., and the dotted line EGFH is the section corresponding to CD.
This construction, by making the external seam straight, leaves no lodgment tor water, • and looks much fairer.to the eye ; but it appears to us that it does not give such firm hold when the mast is bent in the direction EH. The exterior parts are most stretched and most compressed by this bending ; but there is hardly any abutment in the exterior parts of these tables. In the very axis, where the abutment is the firmest, there is little or no differ ence of extension and compression.
But this construction has an advantage, which we imagine much more than compensates for these imperfections, at least in the particular case of a round mast : it will draw together by hooping in comparably better than any of the others. If the cavity be made somewhat too shallow for the pro minence of the tables, and if this be done uniformly along the whole length, it will make a somewhat open seam and this opening can be regulated with the utmost exactness from end to end by the plane. The heart of those vast trunks is. very sensibly softer than the exterior circles : Therefore, when the whole is hooped, and the hoops hard driven, and at considerable intervals between each spell—we are confident that all may be .compressed till the seam
disappears; and then the whole snakes one piece, much stronger than if it were an original log.of that size, because the middle has become, by compres sion, as solid as the crust, which was naturally firm• er, and resisted farther .compression. We verified this beyond a doubt, by beeping a built stick of timber which has thisinequality of firmness in a re markable degree, and it was nearly twice as strong as another of the same size.
Our mastmakers are not without their fancies and whims ; and the manner in which our masts and yards are generally built up, is not near so simple as fig. 23.: but it consists of the same essential parts, acting in the very same manner, and derives all kis Amy from the principles which are here em ployed.
This oonstruction isparticularly suited to the si tuation and office of a ship's mast. It has no bolts ; or, at least, none of any magnitude, or that make very important parts of its construction. The most violent strains perhaps that it is exposed to, is that of twisting, when the lower yards are close braced up by the force of many.men acting by a long lever. This form resists a twist with peculiar energy : it is therefore an excellent method for building up a great shaft for a mill. The way in which they are usually built up is by reducing a central log to a ‘polygonal prism, and then filling it up to the in tended size by eta:sting pieces of timber along its sides, either spiking them down, or cocking them into it by a feather, or joggling them by slips of bard wood sunk into the central log and into the slips. N.B. Joggles of elm are sometimes used in the middle of the large tables of masts ; and when sunk into the firm wood near the surface, they must contribute much to the strength. But it is very necessary to employ wood not much harder than the pine ; otherwise it will soon enlarge its bed, and became loose ; for the timber of these large trunks is very soft.
The most general reason for piercing a beam is to increase its length. This is frequently n , in order to procure tie-beams for very wide Two pieces must be• scarfed together.—Numberless are the modes of doing this ; and almost every mas ter carpenter has his favourite nostrum.• Some of them are very ingenious : But here, as in other cases, the most simple are commonly the strongest. •We do not imagine that any, the most ingenious, is equally strong with a tie consisting of two pieces of same scantling laid over each other for a certain length, and firmly bolted together. We acknowledge .that this will appear an artless and clumsy tie beam ; but we only say that it will be stronger than any that is more artificially made up of the same thickness of timber. This, we imagine, will appear sufficiently certain.