OAK TIMBER. 139 



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 substance, 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, etc., 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 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 cylin- 

 drical stem the variation is from 0-8 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 



