318 
TIMBEE 
one with twenty weighed less than one with ten, and 
another with five weighed 2 lbs. 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 facj; 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 Eailway Superintendents of 
Bridges and Buildings recommended the following factors 
