238 FIKST BOOK OF FOEESTKY 



external conditions and with it the dimensions of the piece. 

 Though an advantage in a barrel or tube, by making it 

 more secure against leakage, this peculiarity of wood is 

 nevertheless a drawback not belonging to the metals, but 

 corresponding to the drawback in the use of metals 

 occasioned by their annoying expansion and contraction 

 due to change of temperature. 



Wood decays ; iron and steel oxidize or rust. Both are 

 serious drawbacks to the use of these materials ; but since 

 decay depends on living organisms, whose multiplication 

 is sometimes extremely rapid, at other times almost 

 imperceptible, varying with the conditions of the wood 

 (moisture, temperature, etc.), the decay of woods is gen- 

 erally more damaging than the oxidation of metals. 

 Under water, wood lasts longer than steel or iron. 



HOW TO DISTINGUISH OUR COMMON TREES 



Most of us know a few trees sufficiently well to distin- 

 guish one kind from another. Usually it is the general 

 appearance, sometimes the bark or leaf, which we recog- 

 nize; and often we know the tree only while in its 

 summer dress. 



Let us examine our common trees more closely and 

 note wherein they differ. 



Here is a red-oak tree. Breaking off a twig, we notice 

 that the leaves are simple and not made up of several 



