This being premised, the following tabulated re sults, will, it is presumed, be found highly valuable, and for which we are principally indebted to Mr. Tredgold, who has collected and arranged most of them in his edition of Buchanan's Practical Essay on Mill Work, and in his Elementary Principles of Carpentry.
The mean of these experiments give about 851 tons for the strength of a double bolt of I3 inch diameter, which is about 241 tons per square inch, whereas from a mean of the experiments at Messrs. Brunton's and at Capt Brown's manufactory, the strength in the simple bars is about 274 tons per square inch, hence the strength of iron manufactured into chain without stays to the link is to that in the simple bolt at 241 to 27, viz. it loses about I I per cent of its strength ; but when protected by stays, the loss is sca rcely two per cent.
In addition to the above table of the cohesive powers of different simple materials, we may add the follow ing on the strength of iron and hemp, when manufac tured into chain and rope.
The above are, we believe, the most extensive and best authenticated results relative to the strength of the direct cohesive powers of materials any where collected, and will, we are persuaded, be highly useful to persons having occasion for any reference of this kind, for by a general comparison of all that has hith erto been given on this subject, we have been enabled to detect the erroneous results, (generally arising out of the employment of too small specimens) and have not introduced them into the tables.
It will of course be seen, that in this simple strait the resistance is proportional to the area of section, and that the strength of any sized rod of any materi al given in the table may be found by multiplying the area of section in inches by the strength per square inch, as given in the table.
Of the Resistance of Materials to Compression, or to a Crushing Force.
The exciting force in this case acts directly the re verse of that in the preceding section, but its effect is by no means so simple and defined. There are, in fact, here two cases to be separately considered, one when the body acted upon is too short to bend, and the other when it is of considerable length with regard to its other dimensions. In the former case, the mate rial is destroyed by actual pressure ; but in the latter, it generally bends in one or more directions, and is ultimately broken by a force similar to that exerted by a transverse strain ; it will be better therefore to reserve the latter case till we have treated of the trans verse strain, and confine ourselves here merely to the crushing force, on which subject the following valu able table of experiments have been published by John Rennie, Esq. in the Philosophical Transactions
for 1818.
In these experiments, after the metals had been com pressed to a certain extent, the resistance is stated to have been enormous.
The above experiments, although they will cer tainly be found useful in many inquiries, are not so valuable to an engineer as those given in our first sec tion, because we cannot in the same way establish a general rule to derive from them the resisting power of similar materials in similar blocks. In the former case, there can be no doubt that the strength varies directly as the section, but in this it varies in a much higher ratio, and we are unable from theoretical con siderations alone to ascertain what that ratio is. It is obvious, that as we increase the base of our speci mens, the interior particles in granulated substances, and the fibres in fibrous bodies, are protected from yielding by the lateral resistance of the exterior ones ; and to what extent this proceeds as we increase our dimensions, it is impossible to estimate, because so much depends upon the internal structure of the body. By comparing the first set of Mr. Rennie's experi ments on cast iron with his third, fourth, or fifth, it would appear that the resistance is as the cube of the side; but we can by no means lay this down as a gen eral law.
When wood is submitted to this strain, its destruc tion takes place by separating the fibres from each other, the lateral adhesions being altogether destroyed, (the pressure being endways of the grain.) Mr. Smart, the ingenious inventor of the hollow masts, and the patentee for the compression of wood in the forma tion of casks, canteens, saddle-trees, Sze. has several curious specimens of compressed wood, in which the fibres are reduced nearly to the state of a painter's brush, by the separation of them as above stated.