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Report on the present State of our Knowledge respecting the 

 Strength of Materials. By Peter Barlow, Esq., F.R.S., 

 Corr. Memb. Inst. France, Sfc. Sfc. 



The theory of the strength of materials, considered merely as 

 a branch of mechanical or physical science, must be admitted 

 to hold only a very subordinate rank ; but in a country in which 

 machinery and works of every description are carried to a great 

 extent, it certainly becomes a subject of much practical im- 

 portance ; and it was no doubt viewing it in this light which led 

 the Committee of the British Association, at their last Meeting, 

 to do me the honour to request me to furnish them with a 

 communication on the subject. In drawing my attention to this 

 inquiry, the Committee have subdivided it into the following 

 heads : — 1. Whether, from the experiments of different authors, 

 we have arrived at any general principles ? 2. What those 

 principles are ? 3. How modified in their application to dif- 

 ferent substances ? And what are the differences of opinion 

 which at present prevail on those subjects ? 



To these questions, without a formal division of the Essay, 

 I shall endeavour to reply in the following pages, by drawing 

 a concise sketch of the experimental and theoretical researches 

 which have been undertaken with reference to these inquiries. 



The subject of the strength of materials, from its great prac- 

 tical importance, has engaged the attention of several able 

 men, both theoretical and practical, and much useful informa- 

 tion has been thereby obtained. As far as relates to the me- 

 chanical effects of different strains, everything that can be 

 desired has been effected ; but the uncertain nature of mate- 

 rials generally, will not admit of our drawing from experiment 

 such determinate data as could be wished. Two trees of the 

 same wood, grown in the same field, having pieces selected 

 from the same parts, will frequently differ from each other very 

 considerably in strength, when submitted to precisely the same 

 strain. The like may be said of two bars of iron from the 

 same ore, the same furnace, and from the same rollers, and 

 even of different parts of the same bar ; and so likewise of 

 two ropes, two cables, &c. We must not, therefore, in ques- 

 tions of this kind, expect to arrive at data so fixed and deter- 

 minate as in many other practical cases ; but still, within cer- 

 tain limits, much important information has been obtained for 



