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The Strength of Materials is a subject of the very first importance to Engineers and others engaged in the Industrial Arts, forming as it does the basis of all constructive calculation. The absence of reliable Rules, or the misapplication of even correct ones, imperfectly understood, may lead to serious consequences ; on the one hand to a useless excess of strength involving heavy pecuniary loss : or on the other hand to inadequate strength, which may issue in disastrous failure.

Two special objects have been kept in view throughout this work: 1st, that the Rules and Data shall be correct, and therefore trustworthy, and 2nd, that their application to practice shall be clearly understood ; for which purpose, every Rule has been illustrated by examples worked out in detail.

To effect the first object, every Rule has been subjected to the test of experience; almost every available experiment having been examined and compared therewith, the error, or rather the difference per cent. between the Rule and Experiment being given in each case. When the theoretical laws did not bear that test, they were relentlessly modified, or abandoned altogether in favour of Empirical Rules whose accuracy was proved by experiment, although they did not admit of a theoretical demonstration. In that case the great object has been so to modify the Rules that the mean results of calculation should practically agree with the mean results of experiment; and this is all that can be done, for the natural variableness in Materials, will always preclude perfect and uniform coincidence.

The authorities for the experimental Data, &c., are given as they occur, but the wonderfully refined and exhaustive labours of Mr. Hodgkinson should be more particularly mentioned. It is matter for regret that he did not fully analyse his own experiments, nor deduce from them all they were capable of teaching, as for example those on the important subject of the Wrinkling Strain in Plate-iron Beams and Pillars. This omission is supplied to some extent,-however imperfectly, in the present work. NEWTON-ABBOT, DEVONSHIRE,

March, 1883.

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