September io, 19 14] 



NATURE 



53 



the construction of mechanical models and diagrams 

 to explain their views, as in the Lodge cog-wheel 

 diagrams of the aether, the planetary systems of 

 atoms of J. J. Thomson and Rutherford, and the 

 grouping of elements by Soddy. 



Engineers have not the same great difficulties 

 which confront those who are advancing the boun- 

 daries of pure science ; their models are very much 

 what they please to make them ; but, even then, 

 problems arise which are sufficiently difficult to tax | 

 all the resources of applied science. The behaviour j 

 of models considered as similar structures is, there- I 

 fore, a subject which engineers are bound to investi- I 

 gate in order to determine the effects of fixed and | 

 moving loads, the action of wind, the pressure and ( 

 frictional effects of steam and other fluids, and many 

 other problems. 



In. the majority of cases the simplest and most 

 direct method is the experimental study of a model, 

 from which to obtain the data required for cal- 

 culating effects on a full-sized structure, and hence 

 the laws of similarity have received a very close 

 scrutiny. 



Although most valuable information can be ob- 

 tained from models, their usefulness is clearly limited. 

 The effects of the dead weight of a structure are 

 proportional to the cube of the linear dimensicms, and 

 are, therefore, not usually measurable on a model 

 except in exceptional circumstances, as, for instance, 

 where elastic jellies are employed, as in the well- 

 known investigations of Pearson on the stress dis- 

 tribution in reservoir dams. Nor are questions of 

 stability easy to solve, since the forces producing in- 

 stability are proportional to the size of the model. 

 On the other hand, stress effects due to applications 

 of load may be measured by the strains produced in 

 a model of the same material, if the loads are pro- 

 portional to the squares of the linear dimensions. 

 The effects of applied load are studied even better 

 in a model constructed of transparent material, since 

 the variation of stress from point to point can be 

 studied w-ith much greater ease and certainty. 



As detailed models of this latter kind present some 

 variations from the usual laws of similarity, it may 

 be of interest to indicate their nature. Questions of 

 deformation clearly involve the elastic constants of the 

 transparent material and their relation to those of 

 the proposed structure, while stress distribution in 

 the solid is influenced by the value of Poisson's ratio. 

 This latter effect is quite small for glass, but may 

 become appreciable with other substance. It is 

 negligible in a model of any material which approxi- 

 mates to a thin plate stressed by forces in its own 

 plane. 



The optical effects for any given load are, moreover, 

 independent of the thickness of the material, and 

 depend upon the stress difference, so that colour 

 effects are obtained which may be regarded as pic- 

 tures of shear stress throughout the model. Modern 

 researches on ductile materials like structural steel 

 indicate that such materials fail at some limiting 

 value of shearing stress, and since the place where 

 these limiting values are reached in the model are 

 visible to the eye, the weak places in the design of 

 a structure can be ascertained and a faulty design 

 corrected by purely experimental means. 



In this connection it is of interest to mention that 

 M. Mesnager, the chief engineer of bridges and roads 

 to the French Government, has recently constructed 

 an elaborate model in glass of a design of an arched 

 bridge of about 310 feet span. This investigation 

 was considered advisable for a work of this magni- 

 tude constructed of reinforced concrete, in order to 

 check the calculations, especially of maximum stresses j 



NO. 2341, VOL. 94] 



in the arched ribs, which latter were assumed to be 

 fixed at the ends. 



The effects of reinforcements were allowed for by 

 determining equivalent sections of glass for the 

 members of the model. Many difficulties had to be 

 overcome in the production of a model free from 

 optical defects, but these were all successfully sur- 

 mounted. The stresses in the model were determined 

 by aid of a Babinet com}>ensator, and formed a valu- 

 able check upon the calculations for a structure of 

 this great magnitude and somewhat unusual design. 



In this brief and incomplete account of a small 

 branch of applied science relating to engineering the 

 fundamental importance of discoveries in pure saence 

 is manifest. 



The discoveries in pure science and their innumer- 

 able applications to practical ends are ever a potent 

 factor w^orking for the common good, and the value 

 which the British Association places upon applied 

 science was most cordiedly voiced by Prof. Bateson 

 in his Portsmouth address when he said: "To the 

 creation of applicable science the very highest gifts 

 and training are well devoted," and, "The man who 

 devotes his life to applied science should be made to 

 feel that he is in the main stream of scientific pro- 

 gress. If he is not, both his work and science at 

 large will suffer. The opportunities of discovery are 

 so few that we cannot afford to miss any, and it is 

 to the man of trained mind, who is in contact with 

 the phenomenon of a great applied science, that such 

 opportunities are most often given"; and, again, 

 "If we are to progress fast there must be no separa- 

 tion between pure and applied science. The practical 

 man with his wide knowledge of specific natural 

 facts, and the scientific student ever seeking to find 

 the hard general truth which the diversity of Nature 

 hides — truths out of which any lasting structure of 

 progress must be built — have everything to gain from 

 free interchange of experience and ideas." 



Engineers who are more immediately concerned 

 with the problems of directing the great sources of 

 power in Nature for the use and convenience of rnan 

 are indeed grateful to our president for these inspiring 

 words, and trust that the ties which unite investiga- 

 tors in pure and applied science will never slacken, 

 but will knit together more closely for a joint advance 

 to a more perfect understanding and utilisation of the 

 laws of Nature. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 

 Sheffield. — The council has decided to invite Dr. 

 J. B. Leathes, F.R.S., professor of pathological chern- 

 istry in the Universit>^ of Toronto, to accept the chair 

 of physiology rendered vacant by the acceptance of 

 Prof. J. S. Macdonald of the chair of physiology in 

 the University of Liverpool. 



A COPY of the calendar for the forthcoming session 

 of the Citv of Bradford Technical College has been 

 received. It provides full particulars of the various 

 day courses arranged, prospectuses of the evening 

 courses in different departments, and general guidance 

 for intending students. The diploma of the college is 

 awarded to each day student who has been in attend- 

 ance for three complete sessions subsequent to passing 

 the entrance examination, and has passed the college 

 examinations in ali subjects of the diploma course 

 taken. The diploma courses in chemistry and in 

 chemistry and dyeing extend over four years. The 

 diploma courses afford a full training for the various 

 branches of the textile, chemical, and engineering in- 

 dustries. .Arrangements have been made with the 



