6o TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



The deflectometer was calibrated at frequent intervals during 

 the progress of the tests by means of a cathetometer, and 

 maintained its accuracy and freedom from backlash during the 

 whole time. 



From the data obtained in each test, the maximum stresses 

 in the outer skin both for the limit of elasticity and for the 

 rupture load were calculated, and the modulus of elasticity ; in 

 addition a stress-strain diagram was drawn for every test, and 

 it was from these diagrams that the limit of elasticity was 

 determined, though it was usually quite easy to fix the limit 

 by mere inspection of the readings obtained by the deflecto- 

 meter. Figures i and 2 (Plate V.) are typical stress-strain 

 diagrams for Scots pine and ash respectively. 



Sketches were made of each fracture ; in every case rupture 

 began on the tension side and spread across the beam, some- 

 times as a pure tear as far as the neutral plane, and thereafter 

 the tear became very irregular until it reached the outer com- 

 pression face. 



Compression Tests. 



From one of the unstressed cut-off ends, two specimens were 

 prepared for compression tests. The specimens were carefully 

 turned to a standard size, each specimen being 2 inches in 

 height and ||^ inch in diameter. The specimens were tested 

 between flat steel plates with spherical backs, so as to ensure 

 as far as possible a direct axial thrust on each specimen ; in 

 every case the load was gradually increased until collapse 

 occurred, which always took the form of failure by shear stress, 

 the plane of failure making an angle with the vertical axis 

 which varied through fairly wide limits. 



Sometimes shear occurred simultaneously on two planes, the 

 planes in this case being either parallel to one another, or the 

 two planes intersected one another and the angles of the 

 inclinations of the planes to the vertical axis were equal. 



Immediately the tests of one set of specimens were completed 

 they were sent off, again in the airtight tin, to the Botanical 

 Laboratories for determination of the percentage of moisture, 

 which, naturally, varied from that of the beam from which they 

 had been cut. 



The stresses obtained from the actual tests in the case of 

 Scots pine and other softwoods can be reduced, if it is thought 



