The timber principally used in the construction of carriages is the ash. This is not an elastic, but rather a tough and fibrous wood, capable of altering its form by the application of pressure, and therefore when not in large masses requires iron plates to secure it. By boiling it becomes very pliable, and may be formed into almost any shape, provided that it is not too thick. For this purpose it is better to use steam than boiling water, as the latter is likely to dissolve and carry off the gluten which unites the fibres, thus rendering the timber useless. Some ash timber is white at heart, and some red; the white is usually the strongest and best. Some trees which have been grown on hillsides much exposed to constant winds present a remarkably wrinkled appearance through their whole length, and it is scarcely possible to plane their timber smooth; this is the toughest of all ash timber. Parts of ash-trees are sometimes found of a yellowish-brown colour, accompanied by a fetid acid smell. This is sometimes attributed to the effect of lightning, but more probably it is a putrid fermentation of the sap, owing to imperfect drying. All other circumstances being equal, the timber is best which is cut down when the circulation of the sap is slowest, as the pores are then open. In the process of drying or seasoning the bulk diminishes considerably. One of the qualities which render ash peculiarly fit for carriage construction is the absence of elasticity, and consequent indisposition to alter its form by warping or twisting. It is not well adapted for boards or planks in which much width is required, as in drying it cracks a great deal. The diameter of ash-trees used by carriage-builders varies from 1 foot to 3 feet 6 inches. It should be borne in mind in cutting ash, that the interior and the outer casing under the bark are rather softer and less durable than the parts between them. Beech is sometimes used by carriage-builders and by wheelwrights, on account of its cheapness; but it is very liable to warp and rot, and consequently unworthy of the attention of the conscientious manufacturer. Elm is largely used for planking where strength is required. The grain is wavy, hard to work, brittle, and apt to split without care. It is not a good surface to paint on, as the grain shows through several coats of colour. It is also used for the naves or stocks of wheels. Oak is used for the spokes of wheels. The best kinds are made from the timbers of saplings, which are not sawn but _cleft_, in order that the grain may be not cut across and render the spoke unfit to resist the strains it will be subject to. Spokes are also made from the limbs of large trees.