525. The silver fir differs from the pines and firs previously described in having its cones erect, which has induced botanists to class it as another genus (Picea), but in other respects the general character is the same.

The tree is a native of Europe, Asia, North America, and it also grows in British plantations. It is generally to be found in regions more temperate than those in which the spruce firs abound.

The silver fir tree is remarkable for the regularity and symmetry of its pyramidal head, and is readily distinguished from the genus Abies by the leaves being more decidedly in two rows, by the cones being upright and having the scales deciduous, i. e. falling off annually.

The tree is large, sometimes attaining a height of more than 100 feet, with stems from 3 to 5 feet in diameter. It produces the Strasburg turpentine of commerce. It is supposed to attain its greatest perfection in this country in about eighty years, and the average increase during that period has often been a cubic foot of wood for each year.

The girth of a tree at Woodhouselee, Midlothian, was 7.4 feet in 1759, at 4 feet from the ground, and in 1793 it girted 11.12 feet.

The wood is of a good quality, and much used on the Continent both for carpentry and ship-building. The harder fibres are of a yellow colour, compact and resinous; the softer nearly white. Like the fir it is light and stiff, and does not bend much under a considerable load; consequently floors constructed of it remain permanently level. It is liable to the attack of the worm. Wiebeking says it lasts longer in the air than in water, and it is therefore more fit for the upper parts of bridges than for piles and piers. The weight of a cubic foot is about 25 1/2 lbs.