562. Of the Elm-tree there are five species now common in Britain; viz. the common rough-leaved elm, the cork-barked elm, the broad-leaved elm or wych hazel, the smooth-leaved or wych elm, and the Dutch elm.

The Rough-leaved Elm ( Ulmus campestris) is common in scattered woods and hedges in the southern parts of England, and is also found very plentifully in France and Spain. It is a harder and more durable wood than the other species: it resists moisture well, and is therefore preferred for coffins. The tree often attains in England a height of 70 to 80 feet, with a stem of 4 feet in diameter.

563. The Cork-barked Elm (Ulmus suberosa) is very common in Sussex, but the wood is of an inferior kind.

564. The Broad-leaved Wych Elm (Ulmus montana), also called Wych Hazel, from its resemblance to the hazel-tree, is a native of Britain and other parts of Europe, and is more cultivated in Scotland than the English elm. It is also cultivated in Ireland as a timber tree, but in England and on the continent of Europe it is not so much cultivated as the U. campestris. The tree is commonly found growing from 70 to 80 feet high, with stems of from 3 to 4 1/2 feet in diameter.

565. The Smooth-leaved Wych Elm (Ulmua glabra) is common in Herefordshire, Essex, the north and north-east counties of England, and also in Scotland. It grows to a large size, and is much esteemed. It is readily distinguished by its smooth, dark, lead-coloured bark, and by its leaves being nearly smooth on the upper surface. The wood is tough and flexible, and is said to be preferred for the naves of wheels.

566. The Dutch Elm (Ulmus major) is a native of Holland; its wood is very inferior to the other species; indeed, Miller says it is good for nothing.

567. The Wych Elm is the largest tree, and the Dutch Elm the smallest. Hassenfratz states the mean size of the trunk of the elm-tree to be 44 feet in length and 32 inches in diameter. The trunk of the common rough-leaved elm is often rugged and crooked, and the tree is of slow growth. Marshall says the Vale of Gloucester produces some very fine elm-trees, but he has not described the species.

Elm has always been much esteemed for its durability in situations where it is constantly wet; and it is also said to be very durable in a perfectly dry state, but not when exposed to the weather. The piles upon which old London Bridge stood were chiefly of elm, and remained six centuries without material decay; * and several other instances of its durability in water have been noticed.

Elm is not useful for the general purposes of building, but from its durability in water it makes excellent piles and planking for wet foundations. It is also used for waterworks, pumps, and for water-pipes before the introduction of cast iron. The naves of wheels, the shells of blocks for tackle, the keels and sometimes the gunwales of ships are made of elm.

* Hutton's ' Tracts,' vol. i., p. 119.

The colour of the heart-wood of elm is generally darker than that of oak, and of a redder brown. The sap-wood is of a yellowish or brownish white, with pores inclined to red. Elm is in general porous, and cross-grained, sometimes very cross-grained, and has no large medullary rays. It has a peculiar odour. It twists and warps much in drying, and shrinks considerably both in length and breadth. Elm is difficult to work, but is not liable to split, and bears the driving of bolts and nails better than any other timber. The timber of the English elm is generally considered the best; that of the wych elm is equally as good, but the Dutch elm is very inferior.

The cohesive force of a square inch of elm varies from 6070 to 13,200 lbs.; and the weight of its modulus of elasticity for a square inch is about 1,343,000 lbs. The weight of a cubic foot when dry is from 34 to 47 lbs.; and when merely seasoned, from 36 to 50 lbs.

According to the experiments of Mr. Couch, elm shrinks one-forty-fourth part of its width in seasoning.

568. In America the elm is also to be found, and is used for many of the purposes to which the European species are applied.

The common American Elm ( Ulmus Americana) grows in the low woods of North America, from New England to Canada, where it attains a height of 80 to 100 feet. The wood is not considered so good as the European species.

569. The Canada Rock or Mountain Elm (Ulmus race-mosa), so called from the rocky places on which it grows, supposed to he a variety of the last, is common to Canada and the northern states of America. It is used, as a substitute for American white oak, for boat-building in the Royal Dockyards of England, for which it is well adapted; but it is a timber very liable to shrink, and it soon becomes shaky when exposed to the sun and wind. When kept in a dry and confined place, the rock elm will sometimes become so perished as to resemble cork.

The annual rings are very close, and the fibres porous, and the wood is remarkable for its free and clear texture throughout its entire length, even up to 40 or 50 feet. It is therefore very flexible.

The weight of a cubic foot while green is about 55 lbs., and when seasoned about 47 1/2 lbs. It shrinks about the one-twenty-fourth part in width in seasoning.

570. The Slippery Elm (Ulmus fulva) is a native of North America. The wood is of a darker colour than the U. Americana; it is also of an inferior quality, though much used for a variety of purposes.