222 



NOTICES OF AUTHORS. 



that experimenters have not attended sufficiently to 

 the structure and nature of the timber, the position 

 and quality of the different layers, &c. Take, for 

 example, the stem of a tall tree, 100 years old : At 

 the cross section, it is found to consist of a certain 

 number of layers of matured timber, and of sap timber. 

 These layers having been gradually formed^ the ex- 

 ternal, after those more internal have partly dried, 

 and the internal and matured wood being also filled 

 up to more solid consistency than the external, the 

 stem, on being barked, contracts in drying much 

 more externally than internally. As soon as the 

 surface has dried, the outer layers contracting lat- 

 ■4eriy> are not sufficient to surround the undried in- 

 ternal layers, thence split in longitudinal rifts ; and 

 as the drying proceeds inwards, the cracks deepen 

 till they reach nearly to the heart — these rifts, when 

 the timber is thoroughly dry, being generally wider 

 in the sap timber than in the matm*ed, more than 

 in the proportion of the size of the respective circles. 

 This effect of drying is what every body is acquaint^ 

 ed with. 



Besides lateral contraction, there is also a disposi- 

 tion to contract longitudinally by diying, much 

 greater in the external than internal layers. While 

 the tree is undivided, this greater contraction of the 



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