238 FIRST BOOK OF FORESTRY 



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 annopng 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 dej^ends on living organisms, whose multiphcation 

 is sometimes extremel}' rapid, at other times almost 

 imperceptible, varying with the conditions of the wood 

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

 eralh' more damaging than the oxidation of metals. 

 Under water, wood lasts lonu'er 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 



