The theory (so to term it) of Carpentry is found-. ed on two distinct portions of mechanical sciences, i namely, a knowledge of the strains to which fratu-' ings of timber are exposed, and a knowledge of their relative strength.
We shall therefore attempt to bring into one point of view the propositions of mechanical science that are more immediately applicable to the art of Carpentry, and are to be found in various articles of our work, particularly Rooe and STRENGTH OF MATERIALS. From these propositions we hope to deduce such principles as shall enable an attentive reader to comprehend distinctly what is to be aimed at in framing timber, and how to attain this object with certainty : and we shall illustrate and confirm our principles by examples of pieces of Carpentry which are acknowledged to be excellent in their kind._ The most important proposition of general me chanics to the carpenter is that which exhibits the composition and resolution of forces ; and we beg, our practical readers to endeavour to form very dis tinct conceptions of it, and to make it very familiar to their mind. When accommodated to their chief purposes, it may be thus expressed: 1. If a body, or any part of a body, be at once pressed in the two directions AB. AC (fig. I. Plate XLVIII.), and if the intensity or force of those pressures be in the proportion of these two lines, the body is affected in the same manner as if it were pressed by a single force acting in the direc tion AD, which is the diagonal of the parallelo gram ABDC formed by the two lines, and whose intensity has the same proportion to the intensi ty of each of the other two that AD has to AB or AC.
Such of our renders as have studied the laws of motion, know that this is fully demonstrated. Such as wish for a very accurate view of this pro position, will do well to read the demonstration given by D. Bernoulli, in the first volume of the Comment. Petropol., and the improvement of this demonstration by D'Alembert in his Opuscides and in the Comment. Taurinens. The practi tioner in Carpentry will get more useful confi dence in the doctrine, if he will shut his book, and verify the theoretical demonstrations by actual ex periments. They are remarkably easy and convinc-1 ing. Therefore it is our request that the artist, who is not so habitually acquainted with the subject, do not proceed further till he has made it quite familiar to his thoughts. Nothing is so conducive to this
•as the votaid s since this- only re. quirts thetrifiing expence of two small-pulleys and few yards of whipcord, we -hope that none of our -- practical omit-it: They willthank us for this injunction.