Young 1 s Modulus and Poisson's Ratio for Spruce. 871 



may be concluded that the source emitting this spectrum is 

 the seat both of moderately strong electric forces and of some 

 special chemical actions. The temperature of the vapours in 

 this case would be higher than in the preceding one. 



Thus it is evident that if we wish to draw any conclusions 

 with regard to the agents at work in a source of light, we 

 must not be guided merely by the fact that certain types of 

 lines are present or predominate in its spectrum, but we 

 should pay special attention to the character of the lines. 

 Further, it should be necessary to take into account the 

 presence of certain gases, such as hydrogen or nitrogen, 

 and note the character of their spectrum lines or bands. 

 And last, but not least, notice should be taken as to whether 

 the luminous phenomenon observed is permanent or only 

 transitory with rapid spectral changes ; for we have seen that 

 the effects of intense electric fields are most strongly brought 

 out in the early stages of development of a light source (arc 

 or spark) when the temperature and the conductivity of the 

 vapour are still low. As the temperature increases, that 

 emission which depends upon the presence of a high potential 

 gradient diminishes or even vanishes. Thus, it would be 

 dangerous to conclude that a star possesses a high tempe- 

 rature merely because spark lines predominate in its spectrum, 

 and such a conclusion would be specially difficult to maintain 

 in the case of the early stages of development of a new star 

 or Nova. 



Manchester and Paris, 

 July 1921. 



XCY. Young's Modulus and Poissons Ratio for Spruce. By 



EL Cabmngton, B.Sc, M.Sc.Tech., A.M.I.Mech.E* 



[Plate XXII.] 



PREVIOUS research on the elasticity of timber has 

 extended over many species, but has been almost 

 entirely confined to the determination of values of Young's 

 Modulus along the grain f. The only values of Poisson's 

 Ratio which appear so far to have been published are a few 

 for pine, box, and beech, by A. Mallock %. 



* Communicated by the Author. 



t Todhunter and Pearson, 'The Theory of Elasticity.' Unwin, 

 ' The Testing of Materials of Construction.' Johnson, ' The Materials 

 of Construction.' S. J. Record, 'The Mechanical Properties of Wood.' 

 W. H. Baring, ' Timber : Its Identiiication and Mechanical Properties ' 

 (The Royal Aeronautical Society, London, 1918). 



J Proc. Royal Society, vol. xxix. (1879). 



