OAK TIMBER 139 



off all the air and water from the wood, and is found to 

 be l - 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 3 9 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 (O028 to 0*435 per 

 cent.), the effect is very marked in other directions. In 

 the radial direction i.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 i.e. parallel to a tangent to the cylindrical 

 stem the variation is from O8 to 10' 6 per cent. 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. It is said that wood 

 from the variety sessiliflora shrinks more than that of 

 the variety pedunculata, but it may be doubted how far 



