118 OF WOOD IN GENERAL 



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 Eowke, 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. E. A. Campbell, experimenting on 

 Austrahan timbers in 1879, employed a sectional area of only J^ 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 



6= breadth in inches, c?== depth in inches, Z= 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 1 inch 

 square and 12 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 stif- 

 ness 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 wifl have, in order to be equally stiff or rigid, to 

 be 24 inches deep. Strength, on the other hand, in soUd rectangular 

 beams, varies inversely as the length, directly as the breadth, and 

 directly as the square of the depth, so that, in the example given 

 above, the longer beam will only require to be 17 inches deep in 

 order to be as strong as the shorter. If the beams are equal in 

 breadth, but of different length, and are required to be equal in 

 stiffness, their breadths must be as the cubes of the lengths. In 

 two beams 24 and 12 feet long, for example, the breadths must be 

 in the ratio of 24^ to 12^, i,e. 13,824 to 1,728, or as 8 is to 1. In other 

 words, the long beam would have to be eight times as broad as the 

 shorter one to be equally rigid, whereas it only requires to be twice 

 as broad to be equally strong. So, too, in cyhnders, the strength 

 varies as the cube, the stiffness as the fourth power of the diameter. 



The constants or values of deflection were deduced by Barlow 



