Not being altogether satisfied with these results, as the test-pieces 

 did not seem to be of sufficient size to give results which could be cofl- 

 sidered of standard practical value, new holders, with spherical seats, 

 were designed, and are shown in Fig. F. 



With these holders, tests can now hr made upon specimens in which 

 the shearing surface has a width of 8-ins. and a depth limited by the 

 tensile strength of the timber, the maximum shearing area being 06 sq. 

 inches. The web of the specimens is usually about .7 in. in thickness, 

 so that the depth should not exceed .H5 '/, / being the tensile and the 

 shearing strengths in Ibs. per ?<|. in. The depth of the shoulder form- 

 ing the bearing lor the pressure require! to produce the shear is about 

 J inch, iind is made of only sufficient sectional area to resist failure 

 by compression, as the deeper the shoulder the greater will be the 

 bending action introduced. 



From the Tables giving the results of the shearing experiments, the 

 following inferences may be drawn : 



a. The shearing strength of the timbers is much less near the heart 

 than at a distance from the heart. 



b. Generally speaking, the shearing strength increases with the 

 weight per cubic foot. 



c. The shearing strength increases with the density of the annular 

 rings, or rather with the proportion of hard to soft fibre. 



d. A failure sometimes occurs, for which it is difficult to find a com- 

 plete explanation. 



For example, the two specimens from Beam X, and designated in the 

 Table by a *, were precisely similar in dimensions and in weight, and ali-o 

 occupied precisely similar position? relatively to the heart in the stick 

 from which they were cut. One of these specimens failed under a shear of 

 470.24 Ibs. per sq. in., and the other under a shear of 301.84 Ibs. per e<j. 

 in., so that the shearing strength of the latter was more than 35 per cent, 

 lens than that of the stronger specimen. A careful examination ofth 

 surfaces of fracture showed no visible difference in the specimens, and the 

 only possible conclusion to be drawn seems to be cither that one of the 



106 



