318 TIMBEE 



one with twenty weighed less than one with ten, and 

 another with five weighed 2 Ibs. per cubic foot more than 

 one with twelve rings. In the case of pine or fir timber, 

 where the rings are abnormally close, the weight when 

 well dried may be somewhat more than in the case of 

 timber with open rings, but in oak and elm the fairly wide 

 rings form the heaviest wood. 



A piece of water-logged American white pine (the yellow 

 pine of the English market) might easily, although when 

 seasoned its weight would only be about half, weigh more 

 than that of a fairly seasoned piece of jarrah or karri. All 

 timber will sink if left long enough in water, as is proved 

 by the fact that dry sawdust from the softest wood will 

 quickly sink if placed in water, but it might take years to so 

 saturate a log of timber as to cause it to sink ; thus we find 

 water-logged ships floating about the ocean to the danger 

 of navigation sometimes for years. 



If by a large number of tests on fairly large-sized pieces 

 of timber we deduce a fairly accurate modulus of rupture, it 

 would be safe to assume a factor of safety of 4 in the case 

 of ordinary construction work that is, assume the working 

 strength as one quarter the breaking strength ; but in the 

 case of machinery, or in structures carrying moving or 

 jarring loads, a factor of safety of 5 or 6 should be 

 allowed. 



Factors of safety, as at present arrived at, are more or 

 less in the nature of guesswork, and are, as has been said, 

 " an expression of ignorance or lack of confidence in the 

 reliability of values of strength," but with a larger number 

 of reliable tests they may be reduced to a more definite 

 figure and to a minimum. 



Some twelve years ago a committee of the American 

 International Association of Railway . Superintendents of 

 Bridges and Buildings recommended the following factors 



