176 THE BOOK OF NATURE STUDY 



III. TIMBER. 



Visit any saw-mill within reach of the school. Get sections of the different 

 woods being used ; make drawings, showing thickness of the heart wood 

 and of the sap wood. 



At the saw-mill, note the lengths in which the wood is cut, and any process 

 of manufacture that may be carried on there. 



Bark may be obtained from the yard of the saw-mill, and the structure ex- 

 amined. 



IV. EFFECT OF TREES. 



i. On soil ; compare the depth of leaves in different woods, 

 ii. On moisture of the air ; with a hygrometer compare the humidity of air 



in a wood with that of air in the open, 

 iii. On temperature ; trees shelter from cold winds and help to make climate 



more equable, 

 iv. On purity of air ; they increase ozone. 



BIBLIOGRAPHY. 



Schimper, Plant Geography ; Schlich, Manual of Forestry, vol. i. ; Step, 

 Wayside and Woodland Trees; Percy Groom, Trees and their Life Histories ; 

 Marshall Ward, Trees, vol. i., "Buds and Twigs" ; Scott Elliott, Nature Studies. 



