To discover rules which will in all cases determine the exact strength of timber has for many years been an object of interest with scientific men. Mr. Tredgold, an eminent writer on the science of carpentry, has laid down at length the results of the best investigations made by himself and others; but, in summing up, he speaks as follows: "The age of trees at the time of cutting; the natural defects, such as knots, shakes, etc.; also the mode of seasoning, or the comparative dryness, - are the cause of some difference in the strength and stiffness of timber. All these things considered, it is impossible to calculate correctly its strength and stiffness." After reminding the reader that the "precision which is so essential to the philosopher is not absolutely necessary to the architect and engineer," he says, "They content themselves with approximations that are simple and easy to be obtained; and, provided that the limits which cannot be passed with safety be pointed out, these approximations are sufficient to direct their practice." *

Mr. Peter Nicholson (from whose works subsequent authors have borrowed ad libitum) remarks as follows: "On that subtile subject, - the proportional strength of timber, on which I gave some observations and calculations in my ' Carpenter's Guide,' - I was in hopes that I should have been able to reduce the theory of scantlings to an arithmetical rule of consequences certain, and of general application. I have to lament that all my endea-dors, assisted by several gentlemen well versed in mathematics, have hitherto been unsuccessful."

Experiments on the strength of timber have, until a late day, tended but little to reform the science of carpentry. Probably more has been done by bold, and perhaps rash, experiments than by all the works which have been written. It remains a fact, however, that the strength of any piece of timber may be determined with sufficient accuracy for all practical purposes. In the examples of framing published in this work, such dimensions for the several timbers are given as experience has approved; and these examples comprise all that any carpenter can need. But, that he may be fully informed in regard to the strength of the various kinds in common use, tables and rules will be given, exhibiting the principles involved. These tables have been prepared expressly for this work, and are founded on the results of many experiments made on dry and sound wood, grown in either Massachusetts, New Hampshire, or Maine. The specimens were selected as a just average of the respective kinds; and, as the experiments were carefully made, the tables may be considered as reliable.

* Tredgold's "Elementary Principles of Carpentry," art. 68, p. 29.

† "Carpenter and Joiner's Assistant," pp. vi. and vii. (Preface).