OAK TIMBER 139 
off all the air and water from the wood, and is found to 
be 1°56, compared with an equal volume of water taken 
as unity. It is the varying quantities of this wood sub- 
stance, and of air and water in the cavities, which make 
the density of different pieces of oak vary so much. 
(3) The proportion of sap contained in the cavities of 
the vessels, cells, &c., of course differs at different times. 
In the spring, just as the buds are opening, the quantity 
of water increases more and more up to about July, when 
the maximum is attained; the proportion of water to 
solids then sinks until October, when the leaves fall; 
it increases again up to Christmas-tide, and then sinks 
to the minimum in the coldest part of the winter. The 
proportion of water to the total weight of the felled 
wood may vary from 22 to 39 per cent. 
(4) Obviously the loss of water on drying causes 
shrinkage of the wood, and although oak shrinks very 
little in the direction of its length (0°028 to 0°435 per 
cent.), the effect is very marked in other directions. In 
the radial direction—+.e. in the direction of the medul- 
lary rays—it may shrink from 1 to 7:5 per cent. of its 
measurement when first felled; and in the direction 
vertical to this—+.e. parallelto atangent to the cylindrical 
stem—the variation is from 0°8 to 10°6 percent. Of 
course, green oak shrinks much more than seasoned and 
older wood, the process of seasoning being, in point of 
fact, the period of chief shrinkage. Itissaid that wood 
from the variety sessiliflora shrinks more than that of 
the variety pedunculata, but it may be doubted how far 
