858 Mr. H. Carrino-ton on the 



It should be noted that where two experiments were 

 performed on one test piece yielding a value for two of the 

 moduli of rigidity, only one value of the elastic limit and 

 modulus o£ rupture could be obtained. Thus the means of 

 the elastic limits and moduli of rupture usually corresponded 

 with a smaller number of results than the number of experi- 

 ments given at the heads of the columns. 



For each experiment the torque was increased at half- 

 minute or three-quarter-minute intervals, and the amounts 

 of the increases were such as to correspond with from eight 

 to twenty readings up to the elastic limit, so that as a rule 

 the elastic limit was reached in about 10 minutes or less. 



From a consideration of the tables, it will appear that the 

 mean values of the percentages of moisture differ little from 

 12 per cent. This sensibly constant humidity was obtained 

 by keeping the prepared test pieces in the laboratory away 

 from the sun, for about one month before experimenting 

 upon them. 



Photomicrographs of normal, tangential, and axial sections 

 magnified about 40 diameters are shown respectively in 

 PL IX. figs. 3, 4, and 5 for balk A, these being also 

 typical of similar sections for the other balks. All the 

 baulks were straight-grained and free from knots and shakes, 

 and represented the good average spruce which was available 

 for aeroplane manufacture early in the war. 



Relations between the Moduli and the Pliysical Properties. 



At the commencement of the research only balk A was 

 experimented upon. It was then found that values of a 

 modulus obtained from pieces which had been nearly 

 adjacent to each other in the balk often varied considerably. 

 This suggested that the variations might be connected with 

 the shrinkages, which were accordingly measured. No 

 relations, however, between the values of a particular 

 modulus and the shrinkages or any of the measured physical 

 properties could be found. From an examination of sections 

 of the pieces under the microscope, differences in structure 

 often appeared, and it seemed likely that investigation along 

 these lines might reveal reasons for the variations. Such 

 refinement being impracticable, it was decided to consider 

 the variations as characteristic of the structural nature of 

 the material, and to experiment upon three other balks, with 

 a view to the possibility of finding relations between the 

 means of the moduli and physical properties. 



