48 



THE COMMERCIAL HICKORIES. 



resilience of spokes increase with the weight of the spoke. This rela- 

 tion of weight to strength is true of various species, as is shown clearly 

 in Circular 15 of the Division of Forestry a and confirmed by many 

 more recent tests. 



RATE OF GROWTH. 



Usually woods are selected on the basis of appearance, very largely 

 on the rate of growth, as shown in the cross section. In hickory 

 the wide-ringed wood, often called " second-growth " hickory, is pre- 

 ferred. Figure 14, based on commercial hickory grown in good situ- 



A.: 



^ 



fxxffts 

 qroupito jooipts 



are oitoiifed 



KH HO Bf> UO MO ISO /60 170 00 190 



WEIGHT , GRAMS. 



FIG. 13. Spoke-test chart, showing relation between resilience factor and weight in clear spokes. 



ations, shows that the work or shock-resisting ability is greatest with 

 wide-ringed wood that has from 5 to 14 rings per inch ; is fairly con- 

 stant from 14 to 38 rings, and decreases rapidly from 38 to 47 rings 

 per inch. The strength at maximum load is not so great with the 

 most rapid-growing wood ; it is at a maximum with from 14 to 20 rings 

 per inch, and again becomes less as the wood becomes more closely 

 ringed. 



The natural deduction is that wood of first-class mechanical value 

 shows from 5 to 20 rings per inch, and that slower growth yields poorer 



o Summary of Mechanical Tests upon Thirty-two Species of American Woods. 



