MECHANICAL CARPENTRY iS so called from the principles of mechanics being employed in the construction of truss framing, or other parts of the art ; while CossTitucTivE CARPENTRY SIP 1WS the rules fur cutting and framing the timbers according to the proposed design. See that article.
The mechanical principles of a piece of carpentry are therefi)re first to be considered ; because they must, in some measure, regulate the disposition and size of the timbers in the design, after which they are to be prepared or formed according to the rules of constructive carpentry.
We shall here state a few of the elementary propositions, with the principles of trussing, and offer some observations on the hest forms of bodies constructed of timber work, to be used under various circumstances. And as it is impossible, in complex parts, to give all the minutia; with mathematical precision, this deficiency will, in a great measure, be com pensated by general infbrmation.
The application of mechanical principles to carpentry was first introduced in this country by Professor Robinson, of Edinburgh, in the Encyclopedia Britannica, whose elaborate papers on this subject prepared the way for that investiga tion which has since been so ably followed out by others. In 1814, Mr. 1'. Nicholson drew up a complete article in every department Of carpentry, fin.1:.ees's Encyclopedia,where this branch of the art, as depending upon the principles of mechanics, was particularly inculcated ; and though several of the plates have been engraved and published some years ago, the manuscript relative to this particular branch has been retained for the article Rolm., in this work, to which, indeed, it chiefly pertains.
In this application of mechanical science, Alex. Nimmo, Esq., F. ft. S. E., the celebrated engineer, also published a very neat and well-connected theory of mechanical carpentry, under the article CARPENTRY, in the Edinburgh Encycla podia ; but perhaps the best work extant on this subject is Tredgold's ELEMENTARY PRINCIPLES of carpentry, which contains a mass of the most valuable information, in the most convenient and available form.
Under the article CURB ROOF, of the present Work, the reader will find an investigation of the best forms for a roof, restricted to certain data. The observations on the strength of timber are reserved for that article; but the practical rules derived therefrom will be here introduced preparatory to the genera] design of mechanical carpentry ; and as these will be chiefly applicable to practice, we shall show the rules under their most simple form ; discarding such as, though accurate, would be too complex for common use.
Therefore, the weights that will break each are nearly in proportion to the numbers 9, 12, and 43, leaving out the fractions ; in which it is to he observed, that the number 43 is almost five times the number 9; therefore the third piece of timber will bear almost live times as much weight as the first; and the second piece nearly once and a third the weight of the first piece ; because the number 12 is one and a third greater than the number 9.
The timber is supposed to be everywhere of the same texture, otherwise these calculations cannot hold true.