506 Notices of Memoirs. 



bodies are altered in form by pressure, and every change of form 

 produces some permanent alteration, however slight, either in shape 

 or texture. When any substance is bent, the total deflection is made 

 up of two elements, which may be termed elasticity and set, or tem- 

 porary and permanent deflection. 



The practical importance of this distinction in iron-work has led 

 to valuable investigations respecting the changes produced in that 

 metal by strains. Mr. Hodgkinson and Mr. Fairbairn have prepared 

 tables which give the results of many experiments of this kind. 

 More recently, M. Tresca has investigated the subject, and his paper 

 on the "Flow of Solids," read before the Institution of Mechanical 

 Engineers at Paris in 1867, contains an account of various experi- 

 ments instituted to prove that the behaviour of liquids under pres- 

 sure is only one case of a general law, which may be extended to solids 

 also, being of course particularly conspicuous in those solids which, 

 like iron and lead, possess a marked ductility. 



The geologist is aware that rocks also are capable of both tem- 

 porary and permanent deflection. Contorted rocks, of which a vast 

 number of examples are known, shew that in many cases compact 

 and solid strata have been crumpled up like folds of cloth, while the 

 contained fossils have been occasionally curiously distorted, yet with- 

 out fracture. It is quite unnecessary to cite instances of what is 

 well-known to all students of geology. From the earliest days of 

 the science these phenomena have been referred, and no doubt 

 justly, to slow and long continued pressure. But I am not aware 

 that the matter has ever been investigated with quantitative pre- 

 cision, and the experiments which I am about to quote were accord- 

 ingly directed to this issue — an exact comparison of sudden and 

 continuous strains with reference to the deflection which they can 

 respectively produce. 



The apparatus which has been contrived to prosecute this inquiry 

 is of a simj)le kind. The machine here exhibited is adapted for pro- 

 ducing visible deflection in thin slabs of stone. The lamina is 

 clamped at one end of a block, the length to be bent is regulated by 

 sliding the block along a groove, and a known weight descends upon 

 the free end. Provision is made that the weight shall act upon a 

 knife-edge, which is always perpendicular to the surface of the slab 

 and always applied to the same line. By means of an index, the 

 deflection can be read accurately to hundredths of an inch. 



With this apparatus I have made experiments for some months 

 past, but the process is so slow, several weeks being required for 

 one operation, that the results hitherto arrived at are very limited. 

 I have as yet tested carefully no material except mountain limestone, 

 and there is still much to be done in studying the efiect of small but 

 long continued strains upon that substance. 



The annexed table gives the results of one series of contortion ex- 

 periments. A number of observed facts have been neglected in order 

 to give prominence to the chief point, viz. the difference in the de- 

 flections which may be produced by the sudden application of a con- 

 siderable weight and the prolonged action of slight pressure. The 



