Ill ] THE CLASSIFICATION OF TIMBERS. 43 



Turning our attention now to those features which 

 are more generally characteristic of timbers, it has to 

 be admitted that their employment is a matter of 

 considerable difficulty in some cases, though it is 

 easy enough in others. I will describe some of the 

 principal varieties as we proceed, and give a few 

 illustrations in each case. 



All Conifers and Dicotyledons which form timber 

 are provided with medullary rays, and it has been 

 found possible to make something of the variations 

 they present in different cases Thus, the medullary 

 rays may be few and relatively far apart, as in 

 Labur?mm and Robinia (with 19 or 20 in a breadth of 

 5 mm.), or numerous and crowded, as in the oak (with 

 64 in a breadth of S mm.). Rhododendron maxhmnn 

 has as many as 140 in the same area, according to 

 Nordlinger. Then, they may be very narrow, requir- 

 ing at least a lens for their observation, as in the 

 pines, ebony, horse-chestnut, willows, &c. ; or suffi- 

 ciently broad to be seen at a glance with the unaided 

 eye, as in the oaks and Casuarina, All degrees of 

 breadth from less than 0*005 mm. to i mm. occur, 

 and the attempt has been made to cast them into 

 six groups, or degrees of fineness, but it seems 

 impossible to define all these groups: nevertheless 

 we can speak of fine, medium, and broad rays 



