TIMBER FOR AEROPLANE AND PIT-PROP PURPOSES. 6 1 



advisable, to a standard moisture of 15 per cent, by the use of 

 the following formula due to Bauschinger : — 



S„ = S [1+0-0366 (</>- 00 )] 



where S., is the required equivalent stress for the standard 

 moisture 4>o of 15 per cent., S being the original calculated 

 stress and </> the observed moisture percentage which usually 

 slightly e.xceeded 15. It has not been considered necessary 

 to give these figures in the following tables. 



The following nine kinds of timber were tested : — 



Scots pine . . 116 samples drawn from districts 



all over Scotland. 

 7 samples. 



Norway spruce 

 European larch 

 Douglas fir 

 Sitka spruce 

 Ash . 

 Wych Elm 

 Birch 

 Willow 



3 samples. 



7 samples. 



4 samples. 



8 samples. 



5 samples. 

 30 samples. 



4 samples. 



The number of transverse tests was, therefore, 1S4, and there 

 was about double that number of compression tests. 



The tables which follow (pp. 62-66) give all the more im- 

 portant results obtained in each test. 



Pit-Prop Tests. 



In this research, which was undertaken to ascertain the 

 suitability of Scottish-grown timber for pit-prop purposes, it 

 was decided that the timber should be tested in compression 

 in the condition in which it was received ; the logs were merely 

 sawn to an appropriate length suitable for the 100-ton Wicksteed 

 vertical testing-machine, the ends being carefully dressed to 

 parallelism and the planes of the end faces adjusted to lie 

 at 90° to the vertical axis of the prop ; the actual length of 

 each prop was measured before test, but was intended in every 

 case to be about 40 inches. 



As the props were not of uniform diameter, but in every 

 case tapered gradually from the big end to the small end, the 

 mean cross-sectional dimension was obtained in the following 

 manner. Rubbings were taken of each of the end faces of 



