lOO TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



compressive stresses, with the result that the intensity of the 

 compressive stress varied across any cross-section from a 

 minimum to a maximum, and the maximum stress would be 

 greater than that disclosed by the method of calculation ex- 

 plained above. 



In order to calculate the bending stresses it would be necessary 

 to know how far the line of thrust deviated from the centre of 

 gravity of the cross-section of the prop at the section of greatest 

 eccentricity, and also to know accurately the true shape of that 

 cross-section. After a careful consideration of the whole matter, 

 I decided that although it was possible to measure fairly 

 accurately the eccentricity of the line of thrust it was impossible 

 to determine with any degree of accuracy the true shape of the 

 cross-section, which deviated a good deal in many cases from a 

 true circular section. 



In view of these facts it was decided to adopt the simpler 

 form of calculation given above, and to ignore the bending 

 stresses, any calculation of which could only have given very 

 approximate results. As has been explained before, the cross- 

 sectional area used in the calculations was the mean of that of 

 the top and bottom faces of the prop, and these latter were very 

 carefully obtained by rubbings. 



Pit-props : Compression Tests. 



Scots Pine. District grown : (3) Finzean. 



Material received, February 1920. Material tested, March 1920. 



