184 VEGETABLE SUBSTANCES. 



curved or twisted, as in the bends of a ship, steam- 

 in £2: is unavoidable ; but the heat and moisture to- 

 jrether ahvays weaken that constituent principle of 

 the timber upon which its strength and durability in 

 a great measure depend. 



Ash is a bad timber for any purpose in building, 

 whether for dry or aquatic purposes, because it nei- 

 ther stands moisture nor the weather. Beech also is 

 a bad timber for building, though when wholly under 

 water it is durable. Elm is quite unfit for building, 

 because of its tendency to warp and shrink with 

 drought, and expand with moisture ; but when wholly 

 imder water, it answers well ; and bolts and nails 

 drive better into it than into any other timber. Beech 

 and elm answer remarkably well for the external or 

 lower keels of ships, and also for the planks nearest 

 the keel, as these are seldom exposed to the air: 

 the same qualities fit them for being used as 

 piles in the construction of bridges and harbours ; 

 though they should never be used above the low 

 water mark. 



The different species of poplar, the sycamore, the 

 lime, and the birch are, generally speaking, unfit for 

 building, being neither strong nor durable ; birch is 

 indeed very tough, but it is more subject to worm 

 than almost any other timber, and moisture soon 

 rots it. Lime, however, is much the best timber for 

 carving, and poplar answers very well for the board- 

 ings of stairs. 



The properties of chesnut have already been 

 pretty extensively mentioned. It is probably the 

 best of the building timbers, though it has been un- 

 accountably neglected in this country. Our ances- 

 tors did more justice to it ; and some of the finest 

 specimens of ancient carpentr}', both in scientific 

 construction, and in tasteful execution, are made of 

 it. When old, it is not very easily distinguished from 



