The rational designing of any kind of construction involves a knowledge of - The external forces to be resisted, transformed, or transmitted; The internal stresses resulting there from; The mechanical properties of the materials to be employed to accomplish the objects sought.

Of these three coordinate departments of knowledge the first two are founded on the sciences of mathematics and applied mechanics. The last one, however, does not rest on any deductive science, as this information can only be gained by patient, expensive, and competent research. For this reason the third essential named above has not kept pace with the other two kinds of engineering science; but, on the other hand, it furnishes very much greater rewards to the skilled investigator.

During the past twenty-five years the number of such investigators has increased from a scattering few to hundreds and even thousands, and these are now found in all enlightened nations. The results of their original studies and experiments are pouring in upon us from all countries, in many languages; and no practising engineer can hope to even scan, much less to appropriate and assimilate, more than a very small part of this vast wealth of experimental knowledge. In the following work the author presents to his leaders a condensed and concise summary of such portions of the knowledge now available on this subject as he has found suitable for such a work. He is fully aware of its incompleteness and of its more or less fragmentary character. Yet with all its faults he believes it contains sufficient reliable information, not commonly accessible elsewhere, to justify its publication in this form.


When the work is used as a text-book in schools of engineering the instructor would do well to assign only such portions of Part I as are required to supplement the student's course in applied mechanics; to have his students read Part II if they do not get this information in other ways; to dwell longer and with more care on Part III; and to call attention only to such portions of Part IV as pertains to the particular course the students are taking. In this way the book may be made intelligent and familiar to the student, and so become to him a great and lasting aid in designing, testing and inspecting, without requiring more time than can be devoted to the subject. This course should precede or accompany an experimental course in the testing laboratory, with which all American schools of engineering are now equipped.

An unusual use has been made of stress-diagrams and other forms of graphical representation of facts and laws, no pains or expense having been spared in this direction. So far as possible tables have been omitted and the original tabular data have been incorporated in diagrams. A law of relationship cannot be perceived from data arranged in a tabular form. When plotted to significant arguments the law not only becomes evident at a glance, but when once impressed on the mind through the sense of sight it cannot well be forgotten. To obtain this lasting benefit, however, the diagram must be intelligently read and understood. The reader is urged, therefore, to give great care to the study of all the diagrams which accompany the text on any subject, for, as a rule, the facts, laws, and conclusions to be drawn from them are not fully expressed in the text. The diagrams must be considered as a part of the text, and they should be read with even greater care than is bestowed on the word embodied ideas.

Throughout the book, with few exceptions, both in the diagrams and in the text, the English units of weight and measure (pound and inch) have been employed. The author m of the opinion that until the metric system has been definitely adopted it is best to use the old units, and that a double system of units is confusing. The revising of books to put them in harmony with the decimal system will be but a very small part of the total expense entailed by the formal adoption of that system by our Government. As a very large part of the data given in the diagrams comes from Continental sources, all of which were expressed in the metric system, a great amount of labor was required to bring this material into the English system of units. Even the results obtained from English sources were generally expressed in long tons per square inch, so that this also required reduction to bring it to pounds per square inch.

Some of the author's usages may be regarded as unwarranted innovations. Especially may this be the case in the matter of the new elastic limit, which he proposes for general adoption, and which is discussed in Arts. 13, 261, 262, and 263. The author bespeaks for these articles a careful consideration and also a study of the many stress-diagrams scattered through the book, before his views are condemned. The fact is, something must be done in this matter, as now no one knows what is meant by "elastic limit" without an explanation - which explanation is not usually given.

The relatively large space given to the subject of timber is not more than its importance as a structural material, and the general absence of scientific information on the subject would seem to demand. Probably the reason little has been given on this subject hitherto, in such works as this, is because little has been known. Until the Forestry Division of the U. S. Agricultural Department began the systematic study of timber and timber-trees, some five or six years ago, very little accurate or scientific information was obtainable as to the mechanical and other properties of American timber. The author's intimate connection with these investigations is a further reason why he should here present an adequate account of the work done to date. It has been no part of the authors aim to give working rules for using materials in structures of various kinds, or to propose original specifications to be used in the purchase of materials. He has tried to impart a knowledge of the properties of materials; on what these depend; the ordinary causes of variation and defects, and how these should be discovered; thus making the reader competent to draw his own specifications and to make his own rules.

The latest forms of investigation of metals and building-stones by means of the microscope are briefly treated (the former in Appendix B); and a chapter has been given on the magnetic properties of iron and steel, and the methods to be employed in determining these.