42 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Febbwabv, 



of almost all materials. Thus, for instance, when malleable iron 

 has received its linal conformation by the process termed cold 

 tv'iiginy, that is by hammerinff it till it is cold, tlie outer particles 

 exist in a state of extreme compression, and the internal ones in a 

 state of extreme tension. The same seems to be the case in cast- 

 iron when it is tal<en from the mould in which it has been cast. 

 The outer portions have cooled first, and have therefore contracted 

 while the inner ones still continued expanded by heat. The inner 

 ones then contract as they subsequently cool, aiul thus they as it 

 were pull the outer ones tog-ether. That is, in the end, the outer 

 ones are in a state of compression, and the inner ones in the oppo- 

 site condition. 



The foregoing- principles may serve to explain the true cause of 

 an inipoi-tant fact observed by Mr. Eaton Hodgkinson in his 

 valuable researches in regard to the strength of cast-iron {Report 

 of the Brithh Association for 1837, p. 3«2).* He found that, con- 

 trary to what had been previously supposed, a strain, however 

 small in comparison to that which would occasion rupture, was 

 sufficient to produce a set in the beams on which he experimented. 

 Now this is just what sliould be expected in accordance with the 

 principles which I have brought forward; for if, from some of the 

 causes already jiointed out, various parts of the beam previously 

 to the application of an external force have been strained to the 

 utmost, wlien, by the ap))lication of such force, however small, 

 they are still farther displaced from their positions of relaxation, 

 they must necessarily undergo a permanent alteration in their con- 

 nection with one another, — an alteration permitted by the ductility 

 t>f the material; or, in other words, the beam as a whole must 

 take a set. 



In accordance with the explanation of the fact observed by Mr. 

 Hodgkinson, I do not think we are to conclude with him, that 

 " the maxim of loading bodies within the elastic limit has no foun- 

 dation in nature. " It appears to me that the defect of elasticity 

 which he has shown to occur even with very slight strains, exists 

 only when the strain is applied for the first time; or, in other 

 words, that if a beam has already been acted on by a considerable 

 strain, it may again be subjected to any smaller strain in the same 

 direction without its taking a set. It will readily be seen, how- 

 ever, fnmi Mr. Hodgkinson's experiments, that the term "elastic 

 limit," as commonly eni])loyed, is entirely vague, and must tend to 

 lead to erroneous results. 



The considerations adduced seem to me to show clearly that 

 there really exist two elastic limits for any material, between which 

 the dis))lacements or deflections, or what may in general be termed 

 the changes of form, must be confined, if we wish to avoid giving 

 the material a set; or, in the case of invariable strains, if we wish 

 to avoid giving it a continuous succession of sets, which would 

 gradually bring about its destruction ; that these two elastic limits 

 are usually situated, one on the one side, and the other on the 

 opposite side of the position which the material assumes when 

 subject to no external strain, though they may be both on the 

 same side of this position of relaxation,t and that they may, 

 therefore, with ))ropriety, be called the superior and inferior limit 

 of tlie change of form of the material for the particular arrange- 

 ment which has been given to its particles ; that these two limits 

 ;ire not fi.red for any given material, but that if the change of 

 form be continued beyond either limit, two new limits will, by 

 means of an alteration in the arrangement of the particles of the 

 material, be given to it in place of those which it previously pos- 

 sessed ; and lastly, that the processes employed in the manufac- 

 ture of materials are usually such as to place two limits in close 

 contiguity with one another, thus causing the material to take in 

 the first instance a set from any strain, however slight, while the 

 interval which may afterwards exist between the two limits, and 

 also, as was before stated, the actual position assumed by each of 

 them, is determined by the peculiar strains which are subsequently 

 apjilied to the material. 



The introduction of new, though necessary, elements into the 



* For further int'urmutlun regarding Mr. Hodgkinson's views and experiments, see his 

 fommunications in the " TraiisuctioDS of the Sections of the British Association" for 

 the year lH4.'i (p. 23) and 1844 (p. L'.'-i), and a work by him, entitled •' P^xperloiental Re- 

 searches on tlie Strength and other properties of Cast Iron." 



t 'I hus, if the section of a henni be of some such form 

 as that shown in either of the accompanying fiijures the 

 one rih or the two ribs, as the case may be, being very 

 wetik in comparison to the thick part of the beam, it may 

 readily occur tliat the two elaatic limits of delledioii may 

 be s'tiiat'-d both on the same side of the position assumed 

 by th« beam when free Irooi external force. For if ttie 

 Ijea'ji has been supported at its extremities and loaded at 

 i'B mi'idle till the rib A H hag yielded by its ductility so as 

 to make all its particles exert their utmost tension, and if 

 llie 111 'd he li'iw gradually removed. th.° particles at B may 

 come to be'compre«si-(l Iaj the ulniost belore the load has 

 ijeen entirely rtuiove -. 



Svo. 1B46. 



consideration of the strength of materials may, on the one hand 

 seem annoying from rendering the investigations more com])licated. 

 On the other hand, their introduction will really have the effect of 

 obviating difficulties, by removing erroneous modes of viewing the 

 subject, and preventing contradictory or incongruous results from 

 being obtained by theory and experiment. In all investigations, 

 in fact, in which we desire to attain, or to approach nearly to, 

 truth, we must take facts as they actually are, not as we might be 

 temjited to wish them to be, for enabling us to dispense with ex- 

 amining processes which are somewhat concealed and intricate, 

 but are not the less influential from their hidden character. 



EDUCATION OF ENGINEERS. 



The study of engineering presents peculiar difficulties; not 

 from the paucity of information, but from the multiplicity of its 

 sources, which are so many and so widely separated, that the 

 student may well be bewildered and discouraged when presented 

 with a map of his future course. Let us consider a few of the 

 branches of knowledge with which he must be adequately ac- 

 quainted before he can be said to have mastered the whole of the 

 principles of his profession. 



As a large part of his business consists in adapting mineral pro- 

 ducts to useful purposes, he must be acquainted with the me- 

 chanical and chemical properties of minerals, and must be able to 

 di.stinguish good metal from bad, sound building materials from 

 those which are perishable, &c. He must, therefore, be acquainted 

 with the science of Mineralogy — or, at least, that part of it which 

 is susceptible of pi'actical application. The changes which in the 

 progress of time are wrought in those minerals by affecting their 

 molecular or crystalline structure, the value of different methods 

 of working them, the chemical changes wrought by the atmo- 

 sphere, and the action of foreign substances, are considerations 

 which render indispensable an adequate knowledge of Chemistky. 

 The engineer employed in constructing railways, roads, canals, and 

 harbours, must certainly be acquainted with Geology : for, how 

 shall he estimate beforehand the probable cost of his works, their 

 permanency, or the most judicious mode of carrying them on, 

 unless he know the n.iture of the soils in which he will have to 

 operate, the order of their succession, their relative depths, and 

 stratification ? To estimate the proper form and dimensions of 

 the structures which form part of his works, he must be thoroughly 

 versed in the science of Statics. To determine the most effective 

 agents of mechanical power, and the most economical methods of 

 producing and regulating motion, the knowledge of Dynamics 

 will be required. The operation of the steam-engine, atmospheric 

 railways, the air-pump, the ventilation of mines, &c., are to be 

 understood only by the investigation of tlie principles of elastic 

 fluids — Pneujiatics. The sciences of inelastic fluids — Hydrosta- 

 tics, and Hydraulics, are essential in constructing sea-walls, 

 breakwaters, canals, and docks, in ascertaining the power of 

 water-mills and hydrostatic-engines, in works of drainage and 

 water-supply, and in naval architecture. 



The sciences already enumerated by no means exhaust the list 

 included in the engineer's curriculum. Most of them involve a 

 knowledge of Mathematics, and some of them of its highest 

 branches — in dynamics, for instance, the processes of the Diffe- 

 rential Calculus are involved at every step. Mathematical know- 

 ledge will, moreover, be required in a most important branch of 

 the engineers occupation — surveying, and the measurement of 

 works. To lay down the course of a railway or estimate the cubic 

 contents of an embankment, would be impossible without some 

 knowledge of Trigonometry- and Solid Geo.-heiky. 



The institution of colleges expressly intended for the scientific 

 education of young engineers, is an ample evidence of the general 

 recognition of the value of the abstract sciences for practical pur- 

 poses. There are now three colleges in London, or its vicinity — 

 King College, University College, and the College of Civil En- 

 gineers, at Putney, in which a course of study is adopted for the 

 especial purpose of preparation for the practice of civil engineering. 

 In estimating the value of such institutions, it should be carefully 

 remembered that the knowledge of the engineer is of two kinds — 

 scientific and practical knowledge. The former may be acquired 

 from books in tlie laboratory or college lecture-room — the latter is 

 to be obtained in the workshop, or the canal or railway works. 



The substitution of certainty for conjecture, of demonstration 

 for hazardous and imperfect analogies — these, in fact, are the 

 oljects of colleges ot ci-.il engineering. 'i'he details of the 



