TIMBER FOR AEROPLANE AND PIT-PROP PURPOSES. 



67 



the prop, any bark present being carefully removed near the 

 edge of the face, the areas of each of the two irregular 

 circular areas thus obtained were then measured by a plani- 

 meter and the mean of the two planimeter results was taken 

 as the mean cross-sectional area of the prop. 



The load was applied to the strut through strong steel 

 pressure plates with spherical backs, so as to ensure, as far as 

 possible, axial pressure on the prop ; the load was in each case 

 gradually increased until collapse occurred. 



The two figures on Plate VI. show {a) a prop just as it col- 

 lapsed in the testing-machine, {b) five props after removal from 

 the testing-machine — the fourth from the left end was too large 

 in sectional area to be crushed by 100 tons, the limiting load of 

 the testing-machine. In all, 256 props were tested from sixteen 

 different kinds of timber, but Scots pine, Norway spruce, 

 European larch, and Japanese larch were the most important 

 timbers tested, these five accounting for about 200 props. 



The overall diameters of the props varied from 2| inches to 

 9 inches, the latter dimension being about the limit of size 

 which could be crushed with the 100-ton testing-machine. 



The following table gives the mean results obtained for each 

 of the more important classes of timber : — 



Summary of Compression Tests of Pit-Prop Timber. 



The tables which follow give the more important results for 

 a selected series of the pit-prop tests. 



