136 OF WOOD IN GENERAL. 



200 to 1, that is to say, 1 Ib applied to the end of the long arm 

 of the upper lever will exert a stress of 200 Ibs. on the specimen 

 attached by shackles to the lower one. 



The dimensions of the specimens tested by different experi- 

 menters, whether for breaking weights, tensile strength, or other 

 measurements, have unfortunately varied greatly. In contra- 

 distinction to the long beams just mentioned as used by Bau- 

 schinger and Lanza, Captain Fowke, in testing the New South 

 Wales timbers at the Paris Exhibition of 1855 for breaking 

 weight, etc., used samples 2 inches square and 12 inches between 

 supports. Mr. Laslett used samples of the same sectional area 

 but 72 inches between supports; whilst Mr. F. A. Campbell, 

 experimenting on Australian timbers in 1879, employed a sectional 

 area of only ~ of an inch. 



The term strength, when used absolutely, generally means the 

 breaking weight under a bending test, and in English books is 



expressed in pounds. It is found by the formula --, , where 



b = breadth in inches, d = depth in inches, I = length in feet, and 

 E = the constant or modulus. This constant, in England, means 

 the number of pounds' weight applied in the middle of a bar one 

 inch square and twelve inches between supports required to 

 break the bar. 



When a beam is supported at each end in such experiments as 

 these, the distance to which the middle of the beam is forced 

 down below its original position by the load, is termed its 

 deflection. In solid rectangular beams the deflection varies 

 directly as the load and the cube of the length, and inversely 

 as the breadth and the cube of the depth. The resistance to 

 deflection is known as stiffness or rigidity. If then we require 

 two beams of the same breadth, but of different lengths, to be 

 equal in stiffness, then their respective depths must be in 

 proportion to their lengths. Thus, if the beams are 24 and 12 

 feet long respectively, and the latter is 12 inches deep, the former 

 will have, in order to be equally stiff or rigid, to be 24 inches 

 deep. Strength, on the other hand, in solid rectangular beams, 



