112 Notes on the Properties of Wood. 
Many mechanics require dry wood for their works, but it is 
not seldom that they also use it before it is properly sea- 
soned. 
In former remarks I have alluded already to several influ- 
ences which affect the due seasoning of timber, and from 
these it is impossible to adopt a general rule, because in each 
place surrounding circumstances must be taken into consi- 
deration. Let me, however, refer to the cause of the warp- 
ing of floor-planks, which has its reason, that the air does 
not reach the lower side of the deal. It shows itself stronger 
where the plank is placed at the top, and the lower side has 
been left in its rough state, by which a larger quantity of 
moisture is discharged by the outer side, which causes the 
warping. In exposing various woods to the action of water 
it will be found that the younger stems are more subjected 
to its influence, because they absorb more water than old 
wood. For hydraulic constructions none but well seasoned, 
old hardwoods should be used, and the sapwood should 
also be avoided, because it promotes the decay of the sound 
arts. 
_ The specific weight must next be taken into considera- 
tion. Most of the woods swim in the water. The specific 
weight of wood is found by taking first the dry weight; 
then it is thrown into water until quite saturated, after which 
it is weighed again. -We must then weigh it under water. 
If we then divide with the difference of the second and third 
weights, which represents the weights of the water displaced 
by the wood, into the natural dry weight we obtain the 
specific weight. This answers in general well enough, but 
it is not so correct as the process recommended by Mr. 
Marcus Bull. He justly observes that the wood is liable to 
swell in the water, and after having taken the dry weight 
he covered the piece of wood with a varnish, having the 
exact density of the water. This varnish is composed of 
rosin, having a density of 1079, and wax, having a density 
of 0967. By this varnish the water cannot penetrate into 
the wood. The solid matter of the wood divested of all 
cavities is heavier than water, and its density or weight, 
compared with that of water, varies, according to Rumford, 
from 1°46 to 1°53. 
It is very difficult to give a general figure for the specific 
weight of any kind of wood. Pieces from the same tree 
produce different results, but the following figures give an 
average weight :— 
