TIMBER FOR AEROPLANE AND PIT-PROP PURPOSES. 59 



I inch in cross-section, were cut, careful records being kept of 

 the exact part of the log or plank from which the specimens 

 were cut. The history of the tree from which the logs or 

 planks had been cut was known, and full information was 

 available as to the nature of the soil and the climatic conditions 

 of the district in which the trees had grown. 



Each beam was tested transversely in the following manner. 

 It was first carefully calipered in order to determine accurately 

 the cross-sectional dimensions near the centre, and its total 

 length was measured ; it was then at once weighed, and from 

 these data the weight per cubic foot of the timber was determined 

 for the condition of dryness then present in the timber. From 

 the weighing-machine the beam went at once to the testing- 

 machine, where it was so supported and loaded that under any 

 given load the central 24-inch length was subjected to pure 

 bending stresses and was entirely free from all shear stresses. 

 .\ deflectometer capable of recording accurately ^ .^^ in. 

 deflection was attached to the beam before it was placed in 

 the testing-machine, and the deflectometer was set so as to 

 record the deflection of the central 20-inch length of the beam 

 under any load. The deflectometer was adjusted to read zero 

 when the beam was free from any load — except that due to 

 its own weight — and the load on the beam was then gradually 

 applied in the usual way, increasing by yV ton each time, and, 

 as soon as the beam was carrying the intended load, the 

 corresponding deflectometer reading was observed and booked. 

 As soon as the limit of elasticity had been passed, the deflecto- 

 meter was removed, and another much simpler and stronger 

 instrument was fastened to the beam, by means of which the 

 central deflection could be read directly for every load beyond 

 the elastic limit of the wood up to the actual breaking-point. 



Immediately after fracture the two pieces were removed from 

 the testing-machine ; from the unstressed ends about 4-inch 

 lengths were cut off^, and from these the compression specimens 

 were prepared. The remainder of the fractured beam was at 

 once placed in a tin tube with tightly-fitting lid and sent off 

 to the Botanical Laboratories, where my colleague, Mr W. H. 

 Craib, now Prof. Craib of Aberdeen University, at once carried 

 out a series of tests to ascertain the moisture condition of the 

 timber, and made careful microscopical examination of the 

 cellular structure of the wood to determine if it were sound. 



