STRENGTH OF MATERIALS, we have taken a con cise view of the strains to which timber and other materials are exposed in buildings and other con structions, and have promised our readers to enter upon the subject again more at large in a subsequent part of our work, a pledge which we propose to re deem in this place. At the time that article was written, we could not but regret the poverty of direct information to be obtained as to the absolute experi mental strength of various materials of, common ap plication in the arts, and we expressed our hopes that something of this kind would be undertaken by some of our scientific men, in order that the practical me chanic might be furnished xvith principles on which he might place reliance, and not be left to the mere result of his own experience. Since that time much has been done towards obtaining the data then so much wanted, first by Air. Barlow of Woolwich, who ob tained permission from the navy board to make any selection of timber, and to avail himself of any faci lities which the dockyard at that port might furnish for carrying his views into execution. These results, with a pretty general abstract from most preceding ex perimenters, have been published by him in his "Es say on the Strength of Timber, ,S•c." which contains also a valuable set of experiments made by Thomas Telford Esq., and another by Captain Brown (the in genious inventor of iron cables) on the direct strength of malleable iron. Mr. Tredgold also in his Princi ples of Carpentry, and in his Treatise on the Strength of Cast Iron, has added much important information on these subjects, as has also John Rennie, Esq. in the Philosophical Transactions for 1818, and lastly, Mr. Hodgkinson in volume fourth of the Manchester Memoirs, has given us a detail of various experiments of this kind which cannot fail of being considered highly curious and important. From these sources we shall in the present article endeavour to make such a collection of results as to furnish the practical builder and engineer with data on which be may se curely depend, and rules proper for making the re quisite computation in any new case that may present itself.
The following divisions are generally made of the various strains to which materials are exposed.
1. They may be drawn asunder by a force acting endwise.
2. They may be compressed and destroyed by a force acting also endwise.
3. A bar of any substance may be strained laterally, one part being supported, and the strain applied im mediately at the point of the support, as when a tenon breaks or a rafter fails at the wall. If the material is cast iron or any similar substance, viz. non-fibrous, the direction of the force with respect to the body is important, but in fibrous bodies, as timber, this strain may be considered under two distinct heads; accord ingly as the force acts perpendicular to, or parallel with, the direction of the fibres.
4. A bar or beam may be strained transversely, as in the case of a girder or rafter.
5. It may be twisted, as in the axle of mills, &c.
6. It may be strained by any two or more of these forces combined.
7. A material may also be strained by an internal pressure, as in the case of hydraulic cylinders, pieces of ordnance, water pipes, &c.
We shall consider each of these strains in the order in which they are stated above, and make such addi tions to the results given under the article carpentry, as are supplied by the recent experiments to which we have referred, and as these have generally been made with more precautions, with better means, and on larger specimens, we beg distinctly to state, that where these results differ essentially from the former, there can, in our opinion, be no doubt that the great est confidence is due to the latter, we shall therefore generally omit all early and doubtful results.
I. Of the resistance to extension in length arising front the direct cohesion of the fibres or particles of mat ter.